It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "If, while praying, you become doubtful whether you have prayed three rak'ahs or four, then if your predominant thought is that you have prayed three rak'ahs, stand up and pray one rak'ah, then say salam and perform two prostrations of forgetfulness (sahw), and recite tashahhud and then say salam. But if your predominant thought is that you have prayed four rak'ahs, then recite tashahhud and, before saying salam, perform two prostrations of forgetfulness while sitting, then recite tashahhud again and say salam."