Volume 8, Book 78, Number 664:
Narrated 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.”