Musnad Ahmad 5:107

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, Al-Harith ibn Yazid narrated to us, from Abdullah ibn Zurayr al-Ghafiqi, from Ali ibn Abi Talib, who said: While we were with the Messenger of Allah (peace and blessings of Allah be upon him) praying, he departed while we were standing. Then he returned, and his head was dripping [with water]. He led us in the prayer, then said: "I remembered that I was in a state of major ritual impurity (junub) when I stood for the prayer, and I had not performed ghusl. So whoever among you finds a rumbling in his stomach, or is in a state similar to what I was in, let him depart until he finishes his need or his washing, then let him return to his prayer."