If there was any change to the prayer, I would surely inform you; but I am a human being like you and liable to forget like you. So if I do forget, remind me, and if anyone of you is doubtful about his prayer, he should follow what he thinks to be correct and complete his prayer accordingly, make Taslīm, and then offer two prostrations

It is related in this noble Hadīth that the Prophet (may Allah's peace and blessings be upon him) led the Companions in prayer and prayed more or less than usual. So the Companions (may Allah be pleased with them) asked him whether any change had been made to the prayer. In response, he told them that if there were any change to the prayer, he would certainly keep them informed. Then he explained that he was only a human being like us and that he would forget like we do.

After that, he presented the relevant ruling on a person who forgetfully adds something to prayer or subtracts something therefrom. He, the Prophet explained, should build upon the number of Rak‘ahs of which he is certain, complete his prayer accordingly, offer two prostrations of forgetfulness, and then make Taslīm, thus finishing the prayer.