Al-Lu'lu Wal-Marjan 337 — The Book of Mosques and Places of Worship.

Abu Hurairah Narrated from:

The Prophet صلى الله عليه وسلم led us in the Zuhr prayer, offering only two Rak'â and then (finished it) with Taslîm, went out in front of the mosque and stood there putting his hand over a piece of wood. Abû Bakr and 'Umar were also present among the people on that day, but dared not talk to him (about his prayer). And the hasty people went away, wondering, "Has As-Salât (the prayer) been shortened?"Amongst the people there was a man whom the Prophet صلى الله عليه وسلم used to call Dhul-Yadain (the long-armed). He said, "O Allâh's Prophet! Have you forgotten or has As-Salât been shortened?" The Prophet صلى الله عليه وسلم said, "Neither have I forgotten, nor has it been shortened." They (the people) said, "Surely, you have forgotten, O Allâh's Messenger! The Prophet صلى الله عليه وسلم said, " Dhul-Yadain has told the truth."So the Prophet صلى الله عليه وسلم got up and offered two Rak'a and finished his Salât with Taslîm. Then he said Takbîr, performed a prostration of ordinary duration or longer, then he raised his head and said Takbîr and performed another prostration of ordinary duration or longer and then raised his head with Takbîr (i.e. he performed the two prostrations of Sahw) (and finished with Taslim). ( Sahîh Al-Bukhârî, Hadîth No. 77, Vol. 8)