Sunan an-Nasa’i 1243
The Book of Forgetfulness (In Prayer)
Estimating (what is most likely the case)
It was narrated that 'Abdullah said: "The Messenger of Allah (ﷺ) prayed and did more or less (rak'ahs). When he had said the salam we said: 'O Messenger of Allah (ﷺ), has there been some change concerning the prayer?' He said: 'Why are you asking?' So we told him what he had done. He turned back toward the Qiblah and prostrated two prostrations of forgetfulness, then he turned to face us and said: 'If there had been some change concerning the prayer I would have told you.' Then he said: 'Rather I am a human being and I forget as you forget. If any one of you is not sure about his prayer, let him estimate what he thinks is correct, and complete his prayer on that basis, then say the taslim and prostrate two prostrations of forgetfulness.'"