Musnad Ahmad 30:397

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, who said: The prayer was called, and the Messenger of Allah (peace and blessings of Allah be upon him) came and stood in his place of prayer. Then he remembered that he had not performed ghusl (ritual bath), so he turned away. Then he said: "Stay as you are." So we formed rows. Then he came, and his head was dripping [with water], and he led us in prayer.