Narrated Abu Huraira:

The Prophet (ﷺ) led us in the Zuhr prayer, offering only two rak`at 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 (ﷺ) used to call Dhul-Yadain (the longarmed). He said, "O Allah's Prophet! Have you forgotten or has the prayer been shortened?" The Prophet (ﷺ) said, "Neither have I forgotten, nor has it been shortened." They (the people) said, "Surely, you have forgotten, O Allah's Messenger (ﷺ)!" The Prophet (ﷺ) said, Dhul-Yadain has told the truth." So the Prophet (ﷺ) got up and offered other two rak`at 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).