Sahih Muslim Book 1, Hadith Number 20.

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 Messenger of Allah (may peace be upon him) said: (The superstructure of) al-Islam is raised on five (pillars), testifying (the fact) that there is no god but Allah, that Muhammad is His bondsman and messenger, and the establishment of prayer, payment of Zakat, Pilgrimage to the House (Ka’ba) and the fast of Ramadan.