Sahih Bukhari Volume 001, Book 002, Hadith Number 007.
Narated By Ibn 'Umar : Allah's Apostle said: Islam is based on (the following) five (principles):
1. To testify that none has the right to be worshiped but Allah and Muhammad is Allah's Apostle.
2. To offer the (compulsory congregational) prayers dutifully and perfectly.
3. To pay Zakat. (i.e. obligatory charity)
4. To perform Hajj. (i.e. Pilgrimage to Mecca)
5. To observe fast during the month of Ramadan.