Islam is built upon five pillars

On the authority of Abu Abd ar-Rahman Abdullah ibn Umar ibn al-Khattab (may Allah be pleased with him and with his father), who heard the Messenger of Allah (may Allah bless him and grant him peace) say: "Islam is built upon five: the testimony that there is none worthy of worship but Allah and that Muhammad is His servant and His Messenger, the establishment of the prayer, the payment of zakat, the Hajj, and the fast of Ramadan."

Related by al-Bukhari and Muslim.

[al-Bukhari 8; Muslim 16]