Sahih Bukhari Volume 008, Book 073, Hadith Number 077.

Narated By Abu Huraira : The Prophet led us in the Zuhr prayer, offering only two Rakat and then (finished it) with Taslim, and went to a piece of wood in front of the mosque and put his hand over it. Abu Bakr and 'Umar were also present among the people on that day but dared not talk to him (about his unfinished prayer). And the hasty people went away, wondering. "Has the prayer been shortened" Among the people there was a man whom the Prophet used to call Dhul-Yadain (the long armed). He said, "O Allah's Prophet! Have you forgotten or has the prayer been shortened?" The Prophet said, "Neither have I forgotten, nor has it been shortened." They (the people) said, "Surely, you have forgotten, O Allah's Apostle!" The Prophet said, Dhul-Yadain has told the truth." So the Prophet got up and offered other two Rakat and finished his prayer with Taslim. Then he said Takbir, performed a prostration of ordinary duration or longer, then he raised his head and said Takbir and performed another prostration of ordinary duration or longer and then raised his head and said Takbir (i.e. he performed the two prostrations of Sahu, i.e., forgetfulness)."