Abu Dawud Book 2, Hadith Number 1015.
Narated By Abdullah ibn Mas’ud : The Apostle of Allah (PBUH) offered prayer. The version of the narrator Ibrahim goes: I do not know whether he increased or decreased (the rak’ahs of prayer).
When he gave the salutation, he was asked: Has something new happened in the prayer, Apostle of Allah? He said: What is it? They said: You prayed so many and so many (rak’ahs). He then relented his foot and faced the Qiblah and made two prostrations. He then gave the salutation. When he turned away (finished the prayer), he turned his face to us and said: Had anything new happened in prayer, I would have informed you. I am only a human being and I forget just as you do; so when I forget, remind me, and when any of you is in doubt about his prayer he should aim at what is correct, and complete his prayer in that respect, then give the salutation and afterwards made two prostrations.