Sahih Bukhari

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 Messenger (ﷺ) ! 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