The Messenger of Allaah, sallallaahu 'alayhi wa sallam, said:

“Islaam is built upon five (pillars): testifying that there is no god worthy of worship but Allaah and that Muhammad is the Messenger of Allaah, establishing regular prayer, paying Zakaah, Hajj and fasting Ramadhaan.”

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(Reported by Al-Bukhaari, 8; Muslim, 16).