"All praise is due to Allaah. Peace and blessings be upon the most noble of the Prophets and Messenger, our Prophet Muhammad, and upon his family and all of his Companions.
The intelligent person is left with no doubt that the actions of worship performed in the religion have the same position as the foundations of a building; indeed, they are of the same position as that of the soul flowing through the limbs.
The concern of the Legislator with the actions of worship reached such a level that He joined them with the foundations of belief, to the point that the Prophet صلى الله عليه وسلم said, as Imaam Muslim رحمه الله and others recorded,
إِنَّ بَيْنَ الرَّجُلِ وَبَيْنَ الشِّرْكِ وَالْكُفْرِ تَرْكَ الصَّلاةِ
'Indeed, between the person and the polytheism and the disbelief, is the leaving of the prayer.' 
The basis for this statement is found in the Noble Qur'aan:
'So if they repent and establish the prayer and give the zakaah, they are your brothers in the religion. (In this way) We explain the Aayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) in detail for a people who know.' (Sooratut-Tawbah, 9:11)
Hence, Allaah سبحانه و تعالى made brotherhood in the religion dependent upon establishing the prayer and giving the zakaah, and not merely dependent upon articulating the testimony of faith and abandoning opposition.
Certainly, the acts of worship are the signs and outward expressions of belief. The extent of the clarity of the belief in the soul of man and extent to which faith is alive in his heart will determine how firm he is in enacting the commands of Allaah سبحانه و تعالى.
The peak of worship and the most important action of worship is the prayer. It is one of the obligations of the religion, one of the pillars of the Sharee'ah and represents a comprehensive set of regulations encompassing the whole of Islaam. It is the second pillar of Islaam which actualises, in feeling and in deed, the first pillar: the testimony that none has the worshipped save Allaah سبحانه و تعالى, and that Muhammad is the Messenger of Allaah صلى الله عليه وسلم. The rest of Islaam comes as a result of the person's performing the prayer. For this reason, the best action the Muslim can perform and the one that brings him closest to Allaah سبحانه و تعالى is the prayer. The Prophet صلى الله عليه وسلم said,
اسْتَقِيمُوا وَلَنْ تُحْصُوا وَاعْلَمُوا أَنَّ خَيْرَ أَعْمَالِكُمْ الصَّلاةُ وَلَنْ يُحَافِظَ عَلَى الْوُضُوءِ إِلاَّ مُؤْمِنٌ
'Remain steadfast but [know that] you will not [always] be able to. Know that the best of your actions is the prayer and that none preserves performing the wudoo' except for a believer.' 
Having a love for the prayer, eagerly going to perform it, performing it in the most complete and perfect way, both inwardly and outwardly, is an indication of the amount of love of Allaah سبحانه و تعالى and desire to meet Him that one has in his heart.
Turning away from the prayer, being lazy in performing it, being lethargic in going to it, seeing it as something difficult to perform, praying it individually and not in congregation without a valid excuse are signs of the heart devoid of the love of Allaah سبحانه و تعالى and its not desiring that which is with Him. Al-Hasan رحمه الله commented upon the saying of Allaah سبحانه و تعالى,
'But after them there followed a posterity who missed (or became lax in) the prayers, and followed their lusts. Soon they will face destruction.' (Soorah Maryam, 19:59), by saying, 'They deserted the mosques, busied themselves with everyday affairs and clung to the delights of this world and sins. We seek refuge with Allaah سبحانه و تعالى from that.' 
So if this is the case of those who stopped performing the prayer in congregation, what would be the case of the one who abandons the prayer deliberately? Abud-Dardaa' رضي الله عنه said,
أَوْصَانِي أَبُو القَاسِمِ أَنْ لا أَتْرُك الصَّلاةَ مُتَعَمِّداً، فَمَنْ تَرَكَهَا مُتَعَمِّداً فَقَدْ بَرِئَتْ الذِّمَّةُ
'Abul-Qaasim (the Prophet صلى الله عليه وسلم) enjoined me to never leave the prayer deliberately, for the one who leaves it deliberately is absolved of protection.' 
The obligatory prayers are repeated every day and night five times at the appropriate times for them. Preserving these time is like the servant continually responding to the call of Allaah سبحانه و تعالى and his giving thanks for the favours that Allaah سبحانه و تعالى perpetually bestows upon him. Allaah سبحانه و تعالى said,
'Remember Me and I will remember you, give thanks to Me and do not be ungrateful.' (Sooratul-Baqarah, 2:152)
The Subtle, the All-Aware has completed the ease of performing them by obligating them in congregation, for indeed gathering together for the actions of worship is one of the ways to make those actions easier. Allaah سبحانه و تعالى has made the outcome of preserving the prayers to be the excellence of religion, the rectification of faith and a swift reward - all of which make it very pleasing to the believers and make them praise Allaah سبحانه و تعالى for obligating it upon them. Allaah سبحانه و تعالى has said,
'The mosques of Allaah shall be visited and maintained by such as believe in Allaah and the Last Day, establish the regular prayers and pay the zakaah and fear only Allaah. It is they who are expected to be upon true guidance.' (Sooratut-Tawbah, 9:18)." 
 Saheeh Muslim [Eng. Trans. 1/48 no. 146].
 Musnad Ahmad (5/276-277).
 Al-Jaami' li-Ahkaam al-Qur'aan (11/123) by Imaam al-Qurtubee رحمه الله.
 Reported in the Sunan of Ibn Abee Haatim. [Shaykh Jamaluddin Zarabozo حفظه الله comments: "Allaah knows best, but this is probably a typographical error in the Arabic text of this work. Ibn Abee Haatim رحمه الله is not known for having a collection of hadeeth entitled Sunan. Furthermore, no one seems to ascribe this hadeeth to any of Ibn Abee Haatim's works. In any case, this hadeeth has been recorded in a number of collections of hadeeth with different contexts. The recording by Ibn Maajah, however, has been graded hasan by Shaykh al-Albaanee رحمه الله.]
 Shaykh Saalih Ibn Ghaanim as-Sadlaan رحمه الله, Congregational Prayer (Al-Basheer Publications & Translations), pp. 9-12.
See also: The Importance And Virtues Of Salah (Prayer)