On the authority of Aboo `Abd ir-Rahmaan `Abdullaah, the son of `Umar ibn al-Khattab radiAllaahu 'anhumaa, who said: I heard the Messenger of Allaah (sallAllaahu alayhi wa sallam) say:
Islaam has been built on five [pillars]: testifying that there is no deity worthy of worship except Allaah and that Muhammad is the Messenger of Allaah, establishing the salaah (prayer), paying the zakaah (obligatory charity), making the Hajj (pilgrimage) to the House, and fasting in Ramadaan. [related by al-Bukhaari and Muslim]
Explanation of Hadeeth 3