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From the book Al-Mulakhas al-Fiqhi (A Summary of Islamic Jurisprudence) by Sheikh Saaleh al-Fawzaan.The book of Salaah.
Listen / Download Mp3 Here (Time 52:45)
Allaah’s Messenger (sallAllaahu ‘alayhi wa sallam) said:
“If one of you has doubt in his prayer and he doesn’t know how many (rak’aat) he has prayed – either three or four – then he should cast away the doubt and base it on what he is certain of. Then he should make two prostrations before making the Salaam. So if he prayed five (rak’aat), his prayer will intercede for him. And if he prayed his prayer completely, then in both cases it will serve as a spite against the Devil.”
[Reported by Muslim]