Musnad Ahmad 30:686

Ibrahim ibn Khalid narrated to us, from Rabah, from Ma'mar, from al-Zuhri, who said: Abu Salamah informed me, from Abu Hurayrah, who said: The prayer was called, and the people arranged their rows for the prayer. The Messenger of Allah, peace and blessings of Allah be upon him, came out to us from his house, and he approached walking until he stood in his place of prayer. Then he remembered that he had not performed ghusl (ritual bath), so he said to the people: "Stay in your places." He returned to his house, then came out to us while we were in rows, and he stood in the prayer while his head was dripping [with water], having performed ghusl.