Sahih Bukhari Volume 001, Book 005, Hadith Number 274.
Narated By Abu Huraira : Once the call (Iqama) for the prayer was announced and the rows were straightened. Allah's Apostle came out; and when he stood up at his Musalla, he remembered that he was Junub. Then he ordered us to stay at our places and went to take a bath and then returned with water dropping from his head. He said, "Allahu-Akbar", and we all offered the prayer with him.