Musnad Ahmad 830:48

Yazid narrated to us, Hammad ibn Salamah informed us, from Ziyad al-A'lam, from al-Hasan, from Abu Bakrah, that the Messenger of Allah (peace and blessings of Allah be upon him) began the prayer and said the takbir. Then he gestured to them to stay in their places. Then he entered [his house] and came out while his head was dripping [with water]. He prayed with them, and when he finished the prayer, he said: "I am only a human being, and I was in a state of janabah (major ritual impurity)."