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 prayer and added to his prayer or fell short in it - Mansur said: This (doubt about adding or falling short) arose either with Ibrahim (in narrating) from Alqamah, or with Alqamah from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) - Sayyiduna Abdullah (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) completed the prayer and turned his face towards us, we said: O Messenger of Allah, has something new occurred in the prayer? He said: "No." Then he (peace and blessings be upon him) was told what he (peace and blessings be upon him) had done (that is, the addition or shortfall in the prayer), so he (peace and blessings be upon him) folded his leg, faced the qiblah and performed two prostrations, then (after giving the salam) he said: "If something new had occurred in the prayer, I would certainly have informed you; but I am a human being like you, I forget as you forget. So when I forget, remind me. And whoever of you has a doubt in his prayer, let him consider which is correct, then complete his prayer accordingly, then give the salam and perform two prostrations."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9789