Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) led us in a prayer in which he (peace and blessings be upon him) fell short or added. When he (peace and blessings be upon him) completed the prayer, we said: Has something been added to the prayer? He said: "What is that?" We said: You (peace and blessings be upon him) led us in prayer in such and such a way. So he (peace and blessings be upon him) folded both his legs and performed two prostrations, then turned his face towards us and said: "I am a human being like you; when I forget, remind me. And when one of you has a doubt in his prayer, let him seek out what is correct, then complete it, then perform two prostrations."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9790