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 are in doubt whether you have prayed three rak'ahs or four, but your predominant assumption is that you have prayed four rak'ahs, then recite the tashahhud and, before saying the salam, perform two prostrations of forgetfulness while sitting. Then recite the tashahhud again and say the salam."