Musnad Ahmad 30:2658

Waki’ narrated to us, he said: Usamah ibn Zayd narrated to us, from ‘Abdullah ibn Yazid, the freed slave of al-Aswad ibn Sufyan, from Ibn Thawban, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, went out for the prayer. When he said the takbir, he turned away and gestured to them: "Stay as you are." Then he went out and performed ghusl, then he came while his head was dripping, and he led them in prayer. When he had prayed, he said: "I was in a state of janabah (major ritual impurity), and I forgot to perform ghusl."