"I said to Abu Sa'eed: 'One of us performs Salat and we do not know how (much) we prayed.' So he said: 'Allah's Messenger (S) said: "When one of you performs Salat and he does now know how (much) he prayed then let him perform two prostrations while he is sitting."
Reference: Jami` at-Tirmidhi 396 In-book : Book 2, Hadith 249 English translation : Vol. 1, Book 2, Hadith 396
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