Narrated by Ibn Umar (Radhiallaho anho):
Allah’s Messenger (sallallaahu ‘alaihi wasallam)said, Islam is based on (the following) five (principles):
To testify that none has the right to be worshipped but Allah and Muhammad (sallallaahu ‘alaihi wasallam) is Allah’s Apostle.
To offer the (compulsory congregational) prayers dutifully and perfectly.
To pay Zakat (i.e. obligatory charity).
To perform Hajj.
To observe fast during the month of Ramadan.

Sahih Al-Bukhari Vol. 1 : No. 7