Musnad Ahmad 1153:2353

Ziyad ibn Abdullah narrated to us, he said: Mansur narrated to us, from Mujahid, from Abu Bakr ibn Abd al-Rahman ibn al-Harith ibn Hisham, from Abu Hurayrah, who said: "Whoever is overtaken by prayer while in a state of janabah (major ritual impurity) shall not fast." He said: So I mentioned that to Aisha, and she said: "He does not say anything. The Messenger of Allah, peace and blessings of Allah be upon him, used to wake up among us in a state of janabah, then he would rise and perform ghusl (ritual bath). Then Bilal would come to him and call him to prayer, so he would go out and lead the people in prayer while water was dripping from his skin, then he would remain fasting for that day."