Sayyiduna Ibn Mas'ud (may Allah be pleased with him) narrates that the Prophet (peace and blessings of Allah be upon him) said: "I am also a human being; just as you forget, I too can forget. And if any of you ever becomes doubtful in his prayer, then he should carefully consider and act upon what he thinks is most correct, and after saying the salam, he should perform two prostrations of forgetfulness (sujood as-sahw)."