Narrated Ibn Mas`ud: that Allah's Prophet led them in the Zuhr prayer and he offered either more or less rak`at, and it was said to him, O Allah's Apostle ! Has the prayer been reduced, or have you forgotten? He asked, What is that? They said, You have prayed so many rak`at. So he performed with them two more prostrations and said, These two prostrations are to be performed by the person who does not know whether he has prayed more or less (rak`at) in which case he should seek to follow what is right. And then complete the rest (of the prayer) and perform two extra prostrations.