This disease of the heart and the sin which is known as hasad was one of the first sins ever committed. It was one of the main things that drove Shaytaan away from his Lord. In fact, two aspects drove Shaytaan to the horrendous situation that he is in today: pride and envy.
Shayaan was jealous and envious of Adam عليه الصلاة و السلام because of all of the special blessings that Allaah سبحانه و تعالى gave Adam عليه الصلاة و السلام, including the fact that the angels were ordered to prostrate to Adam عليه الصلاة و السلام and the fact that Adam عليه الصلاة و السلام was given knowledge that Shaytaan was not given. Shaytaan’s response was the classic response of envy and arrogance. He felt that he was better than Adam عليه الصلاة و السلام and did not feel that Allaah سبحانه و تعالى was right in giving such bounties to Adam عليه الصلاة و السلام. The following Qur’anic verse describes part of the situation:
“(Allaah) said: ‘What prevented you that you did not prostrate, when I commanded you?’ He (Shaytaan) said: ‘I am better than him (Adam), You created me from fire, and him You created from clay.’" (Soorah al-A’raaf, 7:12)
In addition, the people with whom Allaah سبحانه و تعالى is displeased and those who have gone astray, the Jews and the Christians, are people of hasad. Hence, any time a Muslim has this characteristic, he is taking on one of the characteristics of the people whom Allaah سبحانه و تعالى despises. Allaah سبحانه و تعالى says in the Qur’aan,
“Many of the people of the Scripture (Jews and Christians) wish that if they could turn you away as disbelievers after you have believed, out of envy from their own selves, even, after the truth has become manifest unto them. But forgive and overlook, till Allah brings His Command. Verily, Allah is Able to do all things.” (Soorah al-Baqarah, 2:109)
And another verse about the Jews and Christians reads,
“Or do they envy people for what Allah has given them of His bounty?” (Soorah an-Nisaa’, 4:54)
When hasad spreads among a people, it is very destructive. It strikes at the very core of the love and compassion that believers should have toward one another. The Prophet said concerning hasad, “Creeping upon you is the disease of the peoples before you: envy and hatred. And the hatred is the thing that shaves. I do not say it shaves hair, but it shaves the religion. By the One in whose hand is my soul,” – or he said: “by the One in whose hand is the soul of Muhammad – you will not enter Paradise until you believe. And you do not believe until you love one another. Certainly, let me inform you of that which will establish such for you: spreading the greetings (of peace) among yourselves.” 
According to Shaykh Ibn Taymiyya رحمه الله, hasad is always accompanied by hatred. This is one of the evils of hasad. First the person is envious of the other person. After some time, this envy develops into hatred. 
Hasad may definitely lead to the breaking up of a Muslim community. This is because, in many cases, hasad is acted upon. In other words, the person hates the bounties others have received and he actively works to remove such bounties. For example, suppose someone wanted to be the Imaam of a mosque but instead that job was given to somebody else. Now out of envy, the first person cannot stand that the other person got the job and he begins to do whatever he can to remove the other person from his position – even if he himself is not going to get that position. He begins to talk bad about the new Imaam. He starts getting a group of people behind him who criticise everything the Imaam does. Soon the whole community is split and fighting over the Imaam and whether or not he should be kept in his position. This kind of actions occurs in many circumstances, not just with the position of Imaam. This is one of the ways by which hasad can truly destroy a community and lead to hatred among the Muslims. 
Allaah سبحانه و تعالى has shown in the Qur’aan how hasad can penetrate what is recognised as one of the stronges bonds in this world, the blood relationship. Yes, hasad can even pit one blood brother against another. That demonstrates how dangerous and damaging this disease is. Allaah سبحانه و تعالى shows two very clear examples of this nature in the Qur’aan. About the two sons of Adam, Allaah سبحانه و تعالى says,
“And recite to them the story of Adam's two sons, in truth, when they both offered a sacrifice [to Allah ], and it was accepted from one of them but was not accepted from the other. Said [the latter], "I will surely kill you." Said [the former], "Indeed, Allah only accepts from the righteous [who fear Him]. If you should raise your hand against me to kill me - I shall not raise my hand against you to kill you. Indeed, I fear Allah , Lord of the worlds. Indeed I want you to obtain [thereby] my sin and your sin so you will be among the companions of the Fire. And that is the recompense of wrongdoers." So the Nafs (self) of the other (latter one) encouraged him and made fair-seeming to him the murder of his brother; he murdered him and became one of the losers.” (Soorah al-Maa’idah, 5:27-30)
As Ibn Katheer رحمه الله pointed out, it was mostly his envy for his brother, that his brother’s sacrifice was accepted and his was not, that led him to that grievous deed of killing his own blood brother. 
Allaah سبحانه و تعالى also recounts the story of the brothers of Yoosuf عليه الصلاة و السلام. They were also jealous and envious of their father’s love for Yoosuf عليه الصلاة و السلام. Therefore, they also plotted to do away with their own blood brother. Allaah سبحانه و تعالى says,
“When they said: "Truly, Yusuf (Joseph) and his brother (Benjamin) are loved more by our father than we, but we are 'Usbah (a strong group). Really, our father is in a plain error. Kill Joseph or cast him out to [another] land; the countenance of your father will [then] be only for you, and you will be after that a righteous people." Said a speaker among them, "Do not kill Joseph but throw him into the bottom of the well; some travellers will pick him up - if you would do [something]." (Soorah Yoosuf, 12:8-10)
Hasad is a sign that a person’s soul is evil and is far away from the level of eemaan that it should achieve. The true believer must love for his brother what he loves for himself. He should be happy when his brother is blessed with something. This is virtually the exact opposite of hasad. In the case of hasad, one wishes for the things that Allaah سبحانه و تعالى has bestowed upon others, while Allaah سبحانه و تعالى has stated,
“And do not wish for that by which Allah has made some of you exceed others. For men is a share of what they have earned, and for women is a share of what they have earned. And ask Allah of his bounty. Indeed Allah is ever, of all things, Knowing.” (Soorah an-Nisaa’, 4:32)
Hasad is a king of opposition or discontent with what Allaah سبحانه و تعالى has decreed. After all, it is Allaah سبحانه و تعالى who bestowed that bounty upon the other person. If a person dislikes what another person has received, then, in reality, he is showing his dislike for what Allaah سبحانه و تعالى has decreed. 
When he gets to that point, as Shaykh Ibn ‘Uthaymeen رحمه الله stated, he is putting his religion into a dangerous and precarious position. He is practically declaring himself a better decision maker than Allaah. 
Furthermore, the jealous person does not know why Allaah سبحانه و تعالى has bestowed that blessing on the other person. Some people get jealous because they see people lying or cheating and then getting what they desire off this world. However, these “bounties” that Allaah سبحانه و تعالى has given them may simply be the source of their own destruction – Allaah سبحانه و تعالى gives them bounties and they continue to disobey Allaah سبحانه و تعالى by lying, cheating and so forth. Therefore, their punishment will be much greater. Obviously, there is no reason to be jealous of such worldly gains whatsoever. One must leave the decrees and the decisions of this world to Allaah and accept whatever Allaah سبحانه و تعالى has decreed, for Allaah سبحانه و تعالى is the All-Knowing, the Just, and the Merciful.
Furthermore, the person cannot change what Allaah سبحانه و تعالىhas decreed. In essence, he is fighting a losing battle. He cannot remove that blessing from the other person unless Allaah سبحانه و تعالى decrees such a thing. However, it was Allaah سبحانه و تعالى that decreed that the other person should receive that blessing. His anxiety and stress will be in vain as everything is according to Allaah’s decree and Allaah سبحانه و تعالى blesses whomsoever He wills.
Hasad also entails a number of negative psychological effects upon the jealous or envious person. As Shaykh Ibn ‘Uthaymeen رحمه الله pointed out, the jealous person may always be in a state of anger and discontent over the bounties that others receive. This is because there is no limit to the number of bounties that Allaah سبحانه و تعالى bestows upon others. The person may drive himself crazy or even kill himself if he always looks at the bounties others receive and he cannot live with them receiving such bounties. 
When one realises all of the negative aspects of hasad and how it is clearly a sign that there is something wrong in a person’s heart, especially a person who claims to be a Muslim or a believer, it is not surprising to know that, in reality, hasad and eemaan cannot coexist in one heart. The Messenger of Allaah صل الله عليه و على آله و سلم said, “Eemaan and hasad do not gather together in the heart of a slave [of Allaah].”  
 Recorded by Imaam Ahmad, with the above wording, and by at-Tirmidhi. After a lengthy discussion, al-Hilaali concludes that the hadeeth is hasan. See al-Hilaali, Eeqaadh, pp. 470-471
 Shaykh Ibn Taymiyyah رحمه الله, Majmoo’, vol. 10, p. 127. Also see Ali ad-Duhaami, al-Hath ala Salaamah as-Sadr (Riyadh: Daar al-Watn, 1416 A.H.), pp. 38-39
 Power and leadership are great trials from Allaah سبحانه و تعالى. They are many times the downfall of otherwise righteous people. Some people want all the Islamic activity to be under their control and leadership. If others of the same community, not directly under their guidance, are doing some good work, they are not happy about it because they want all the good work to come from only themselves. Therefore, they start talking about the others and depreciating their good work. They claim that they are going against the “unity of the community” by not doing their good work under their leadership and control. This is a great disease that has afflicted many of the Muslim communities of the West. It must be recognised and cured by realising that it is part of the evil hasad that the Prophet صل الله عليه و على آله و سلم has warned about.
 Tafseer Ibn Katheer, vol. 2, p. 50
 Allaah سبحانه و تعالى has delegated specific roles for Muslim males and females. Especially in this day and age, with the advent of the feminist movement, many Muslim women seem to be jealous and envious of the role that Islam has given to men. Many of them do not seem to be happy that, in general, it is the man who is out in the workplace while the woman is at home (or, as some of them say, “Stuck at home with the children” while they want to “be like the men”). Many of them do not seem to be happy that the man has been given the responsibility of being the head of the household. They do not accept the fact that it is the husband’s job to provide for and maintain the family, as well as lead the family. Such women must be careful because they must consider who their anger is directed to. Is it not Allaah سبحانه و تعالى who created them as women instead of men? Is it not Allaah سبحانه و تعالى who legislated the rights and responsibilities of both sexes? Do these women not think that Allaah سبحانه و تعالى has done that based on wisdom and that Allaah سبحانه و تعالى is merciful to all of His servants, male and female? However, what is important, and what is often times missing, is for the wife to be treated properly and correctly according to Islamic teachings.
 Shaykh Ibn ‘Uthaymeen رحمه الله, Sharh Riyaadus Saaliheen, vol. 4, p. 704
 Shaykh Ibn ‘Uthaymeen رحمه الله, Sharh Riyaadus Saaliheen, vol. 4, p. 704
 Recorded by an-Nasaa’ee. According to Shaykh al-Albaani رحمه الله, it is saheeh. See Saheeh al-Jaami’, vol. 2, p. 1262
 Commentary on the Forty Hadeeth of Imaam an-Nawawi رحمه الله, vol. 2, pp. 1026-1031