(5) باب بَيَانِ أَرْكَانِ الْإِسْلَامِ وَدَعَائِمِهِ الْعِظَامِ
It is narrated on the authority of ('Abdullah) son of 'Umar, that the Prophet (may peace of Allah be upon him) said:
(The superstructure of) al-Islam is raised on five (pillars), i. e. Allah (alone) should be worshipped, and (all other gods) beside Him should be (categorically) denied. Establishment of prayer, the payment of Zakat, Pilgrimage to the House, and the fast of Ramadan (are the other obligatory acts besides the belief in the oneness of Allah and denial of all other gods).
Reference: Sahih Muslim 16 b
In-book reference: Book 1, Hadith 20
USC-MSA web (English) reference(deprecated numbering scheme): Book 1, Hadith 19