Sahih Bukhari Volume 001, Book 011, Hadith Number 612.
Narated By Abu Huraira : Allah's Apostle went out (of the mosque) when the Iqama had been pronounced and the rows straightened. The Prophet stood at his Musalla (praying place) and we waited for the Prophet to begin the prayer with Takbir. He left and asked us to remain in our places. We kept on standing till the Prophet returned and the water was trickling from his head for he had taken a bath (of Janaba).