Sahih Muslim Book 001, Hadith Number 0018.

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

It is narrated on the authority of ('Abdullah) son of Umar (may Allah be pleased with them) that the Holy Prophet (may peace of Allah be upon him) said: (The superstructure of) al-Islam is raised on five (pillars), i.e. the oneness of Allah, the establishment of prayer, payment of Zakat, the, fast of Ramadan, Pilgrimage (to Mecca).

A person said (to 'Abdullah b. Umar the narrator): Which of the two precedes the other-Pilgrimage or the fasts of Rarnadan? Upon this he (the narrator) replied: No (it is not the Pilgrimage first) but the fasts of Ramadan precede the Pilgrimage.