Once 'Umar ibn 'Abdul 'Azîz (rahimahullâh) was advised,
"O 'Umar, beware of being the ally of Allâh in open,while being His enemy in secret.
If one's nature in open and secret do not equate, then he is a hypocrite,
and the hypocrites occupy the lowest level in the Hellfire."
From amongst the most difficult things one can acquire is sincerity and because of this, the pious predecessors ( as-salah as-salih) would try to block off all their acts of worship from riyaa (showing off) by hiding their good deeds just as they would hide their evil deeds.
They would advise each other with words such as "Do not be an enemy to Shaytaan in public and his friend in private"; or in other words, make sure you are the same person in public as you are when in seclusion. Because of this, they loved to be alone and busy in acts of worship wherein only Allah could see them.
As for those deeds which were public, then the following story would suffice: a student of Abdullah bin Masoud Ø±Ø¶ÙŠ Ø§Ù„Ù„Ù‡ Ø¹Ù†Ù‡ once hosted a blind man for a meal and decorated his meal as if he could see. When the people inquired as to why he was decorating the food to for a man who couldn't even see, he replied by saying "But Allah can see!"
Because of this, one of the best definitions of sincerity is to forget about the creation by constantly looking at the Creator (in terms of gaining admiration for your deeds); thereby not caring whether or not the people see any single act of worship. So when a man gets up to worship, he does not think about the people's thoughts and does not care whether they praise him, or if they criticize him because he only sees Allah in front of him, thereby reaching the level of ihsan
Then he (the man) said,
"Inform me about Ihsan." He (the Messenger of Allah) answered, "It is that you should serve Allah as though you could see Him, and even though you cannot see Him yet He sees you." - Muslim
So when one reaches ihsan, he does not become like those who Ali bin Abi Talib Ø±Ø¶ÙŠ Ø§Ù„Ù„Ù‡ Ø¹Ù†Ù‡ said regarding the insincere:
"The one who shows off has three characteristics: he is lazy when by himself, he is lively and energetic when with others and he increases in his actions when he is praised and decreases in them when he is criticised.
"Because of the difficulty of sincerity, they (as-salaf) considered themselves to be hypocrites as it was said that a Tabi'e met thirty companions of the Prophet and all of them feared hypocrisy for themselves. These were the companions of the Prophet , those men and women who Allah was pleased with and who were pleased with Allah, so how about someone lower than them, people like us who act as if we have been granted Paradise while living on earth. And what is even more astonishing than this is Umar bin al-Khattab's Ø±Ø¶ÙŠ Ø§Ù„Ù„Ù‡ Ø¹Ù†Ù‡ asking Hudhayfah ibn al-Yaman Ø±Ø¶ÙŠ Ø§Ù„Ù„Ù‡ Ø¹Ù†Ù‡ if he (Umar) were among the hypocrites!
So one can never be sincere if he thinks himself to be sincere; since sincerity is to lose the vision of sincerity in oneself. And because of the greatness of sincerity, they (as-salaf) would say "whoever had a single instance in his lifetime which was purely and sincerely for the sake of Allah will be saved" since it has the capacity to purify you from all your sins. One of the salaf once remarked on how happy he would be if he only knew Allah accepted even two rakaat of salaah from him!
Because of their sincerity they lived a pure life, free from anxiety and grief since it was to Allah that they turned to in both good and bad times. And perhaps ibn al-Qayyim rahimullah reflected on this when he said: Ikhlaas and Tawheed are a tree in the heart, its branches are good actions and its fruits are a good life in this world, and eternal comfort in the Hereafter; and just like in Paradise where the fruits whose supply is not cut off, nor are they out of reach, likewise are the fruits of Tawheed and Ikhlaas in this world. [Ibn al-Qayyim - 'al-Fawaid' 292]
So be sincere O servants of Allah and leave the inner cravings for attention for indeed the short-term pleasure attained from people's admiration is not worth the anger and wrath of Allah and He is not unaware of what you do. And if you were to reflect on the people of insincerity, you would realize how pathetic their situation is, for indeed the one who is insincere cannot see anything except for what is in front of them, thereby unable to have high aspirations and unable to undergo hardship for anything worthy.
(30:7} They know only the outside appearance of the life of the world (i.e. the matters of their livelihood, like irrigating or sowing or reaping, etc.), and they are heedless of the Hereafter.And if they could see beyond the outside appearance of this world, they would know with certainty that the only thing that separates them from looking up at the face of Allah is death; and they would never risk insincerity for a miserable gain such as fame in this world for how quick is the passing of this world and how forgetful do people become of each other and finally how useless is the deed that is bereft of sincerity; for indeed the only one Satan is able to lead astray is the one who is insincere.
82: [Iblees] said, "By your might, I will surely mislead them all
83: Except, among them, Your sincere and chosen servants.
Make dua Insh'Ala that Allah grants me Ikhlas