Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 18
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.