Sahih Bukhari Volume 2, Book 22, Hadith Number 321.
Narated By Abu Huraira : The Prophet offered one of the evening prayers (the sub-narrator Muhammad said, “I think that it was most probably the ‘Asr prayer”) and he finished it after offering two Rakat only. He then stood near a price of wood in front of the Mosque and put his hand over it. Abu Bakr and ‘Umar were amongst those who were present, but they dared not talk to him about that (because of excessive respect for him), and those who were in a hurry went out. They said, “Has the prayer been reduced?” A man who was called DhulYadain by the Prophet said (to the Prophet), “Has the prayer been reduced or have you forgotten?” He said, “Neither have I forgotten, nor has the prayer been reduced.” He said, “Certainly you have forgotten.” So the Prophet offered two more Rakat and performed Tashm and then said Takbir and performed a prostration of Sahu like his ordinary prostration or a bit longer and then raised his head and said Takbir and then put his head down and performed a prostration like his ordinary prostration or a bit longer, and then raised his head and said Takbir.