Narrated Ibn 'Omar: Allah's (SWT)  Apostle said: Islam is based on (the following) five (principles):

1. To testify that none has the right to be worshipped but Allah and Muhammad is Allah's Apostle.
2. To offer the (compulsory congregational) prayers dutifully and perfectly.
3. To observe fast during the month of Ramadan.
4. To pay Zakat (i.e. obligatory charity) .
5. To perform Hajj. (i.e. Pilgrimage to Mecca)

(Translation of Sahih Bukhari, Book 2: Belief , Volume 1, Book 2, Number 7)