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 fell short or added in it. When he (peace and blessings be upon him) gave the salam, it was said: O Messenger of Allah, has something new occurred in the prayer? He said: "Had that been so, I would certainly have informed you. I am only a human being; I forget as you forget. So whoever of you prays and falls short or adds, let him seek out what is correct, complete it, and perform the two prostrations of forgetfulness (sahw)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9792