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 the prayer with salam, we informed the Prophet (peace and blessings be upon him). Upon hearing this, the Prophet (peace and blessings be upon him) folded his legs and performed two prostrations of forgetfulness (sahw). Then, turning towards us, he said: "If there had been any new command regarding the prayer, I would certainly have informed you. The fact is that I am also a human being; just as you forget, I too can forget. If I forget, then remind me. And if any of you ever becomes doubtful in his prayer, he should carefully consider and adopt the cautious opinion, and after finishing the prayer with salam, he should perform two prostrations of forgetfulness."