Musnad Ahmed: Hadith 3579

The Messenger of Allah (ﷺ) prayed and I do not know whether he added or omitted something. When he said the salam, it was said to him: O Messenger of Allah, has something new been introduced into the prayer? He said: “No. Why are you asking?" They said: You did such and such in the prayer. He turned to face the qiblah, then he prostrated twice and said the tasleem, then he said: "I am human, I forget as you forget. If one of you is unsure in his prayer, let him try to work out what is correct and proceed on that basis, then prostrate twice.”