Sahih Muslim Book 001, Hadith Number 0019.
Chapter : Concerning the saying of the Apostle; Islam is founded on five (fundamentals).
It is narrated on the authority of ('Abdullah) son of 'Umar, that the Holy 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).