Sahih Bukhari Volume 008, Book 078, Hadith Number 664.

Narated By Ibn Mas'ud : That Allah's Prophet led them in the Zuhr prayer and he offered either more or less Rakat, 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 (Rakat) in which case he should seek to follow what is right. And then complete the rest (of the prayer) and perform two extra prostrations."