Sahih Bukhari, Chapter: Call to Prayers (Adhaan)

Once Allah's Apostle prayed two Rakat (instead of four) and finished his prayer. Dhu-l-yadain asked him whether the prayer had been reduced or whether he had forgotten. Allah's Apostle asked the people whether Dhu-l-yadain was telling the truth. The people replied in the affirmative. Then Allah's Apostle stood up, offered the remaining two Rakat and then finished his prayer with Taslim and then said, "Allahu Akbar." He followed it with two prostrations like ordinary prostrations or a bit longer.