Mishkat al-Masabih 1016

‘Abdallah b. Mas'ud said that God’s Messenger prayed five rak'as in the noon prayer and was asked whether the prayer had been extended. He asked what they meant by that, and when they told him he had prayed five rak'as he made two prostrations after having given the salutation. In a version he said, “I am only a human being like you, foigetting just as you do; so when I forget remind me, and when any of you is in doubt about his prayer he should aim at what is correct and complete his prayer in that respect, then give the salutation and afterwards make two prostrations.” (Bukhari and Muslim.)