Sayyiduna Ibn Mas'ud (may Allah be pleased with him) narrates that once the Prophet (peace and blessings be upon him) led a prayer. I do not remember whether there was some deficiency or excess in it. In any case, when he finished with salam, someone asked: "O Messenger of Allah! Has a new command regarding the prayer been revealed?" The Prophet (peace and blessings be upon him) said: "No, what happened?" The companions (may Allah be pleased with them) said: "You led the prayer in such and such a manner." Upon hearing this, the Prophet (peace and blessings be upon him) folded his legs and performed two prostrations of forgetfulness (sahw). Then, when he finished with salam, he said: "I am also a human being; just as you forget, I too can forget. So, if any of you ever has doubt in his prayer, he should carefully consider and act upon what he thinks is correct, and after finishing with salam, perform two prostrations of forgetfulness."