"The Prophet (S) prayed five (Rakah) for Zuhr, so it was said to him: 'Has the prayer been added to, or have you merely forgotten?' So he (S) performed two prostrations after he had said the Salam."
Reference: Jami` at-Tirmidhi 392 In-book : Book 2, Hadith 245 English translation : Vol. 1, Book 2, Hadith 392
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