Musnad Ahmad 5:215

Abdullah narrated to us, saying: "I found this hadith in my father’s book, and most of my knowledge, if Allah wills, is that I heard it from him." Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Abdullah ibn Lahi’ah narrated to us, Abdullah ibn Hubayrah narrated to us, from Abdullah ibn Zurayr al-Ghafiqi, from Ali ibn Abi Talib, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in prayer one day, then he departed, then he came back while his head was dripping with water. He then led us in prayer, and said: "I led you in prayer just now while I was in a state of major ritual impurity (junub). So whoever experiences what I experienced, or finds a rumbling in his stomach, let him do as I have done."