Sahih Muslim Book 1, Hadith Number 21.

Chapter : Concerning the saying of the Apostle; Islam is founded on five (fundamentals).

It is reported on the authority of Ta’us that a man said to ‘Abdullah son of ‘Umar (may Allah be pleased with him). Why don’t you carry out a military expedition? Upon which he replied: I heard the messenger of Allah (may peace be upon him) say: Verily, al-Islam is founded on five (pillars): testifying the fact that there is no god but Allah, establishment of prayer, payment of Zakat, fast of Ramadan and Pilgrimage to the House.