In the name of Allah, Most Gracious, Most Merciful.

According Buraydah (may God grant him His satisfaction), the Prophet (peace and salvation be upon him) said: "The only agreement that exists between us and them (the Unbelievers) is the practice of prayer. Whoever abandons it is indeed fallen into disbelief. " (Reported by Attirmidhi)

Jabir (may God grant him His satisfaction) reported: "I heard the Messenger of Allah (peace and salvation be upon him):" Between man on the one hand, and associationism and denial on the other, is the renunciation of the practice of prayer. " (Reported by Moslem)

Riyad as-Salihin (Gardens of the Righteous)

Imam Mohieddine Annawawi 631-676
Translation and Commentary by Dr. Salaheddine Keshrid (Tunisia)
Publisher: Dar Al-Gharb al-Islami
Hadith 1: Chapter 109, Page 339, 1077 Number

Hadith 2: Chapter 109, Page 339, 1076 Number

Source: http://www.ayaatdujour.com/