Sahih Bukhari Volume 002, Book 022, Hadith Number 319.
Narated By Abu Huraira : Once Allah's Apostle offered two Rakat and finished his prayer. So Dhul-Yadain asked him, "Has the prayer been reduced or have you forgotten?" Allah's Apostle said, "Has DhulYadain spoken the truth?" The people replied in the affirmative. Then Allah's Apostle stood up and offered the remaining two Rakat and performed Taslim, and then said Takbir and performed two prostrations like his usual prostrations, or a bit longer, and then got up.