Abu Ubaidah bin Abdullah narrates from his father (Sayyiduna Abdullah bin Mas'ud, may Allah be pleased with him) that the Prophet Muhammad (peace and blessings be upon him) said: "When you are in prayer and you are in doubt whether you have prayed three or four rak'ahs, and your predominant thought is that you have prayed four, then after reciting the tashahhud, perform two prostrations of forgetfulness (sajda sahw). Do this while you are sitting and before you say the salam. Then after that, recite the tashahhud again and then say the salam." Some other narrators have transmitted this, but they have not narrated it as a marfu' (directly attributed to the Prophet) hadith, and some other narrators have reported some differences in its wording.