Musnad Abu Ya'la al-Mawsili 150:202

Dāwūd ibn Rashīd narrated to us, Shuʿayb ibn Isḥāq al-Dimashqī narrated to us, ʿAbbād ibn Manṣūr narrated to us, from ʿAṭāʾ: That Masrūq asked ʿĀʾishah, saying: "O my mother, a man wakes up in a state of janābah (major ritual impurity), may he fast that day of his?" She said: "The Messenger of Allah (ﷺ) would wake up in a state of janābah from intercourse, not from a wet dream, during an obligatory fast, not a voluntary one. Then he would perform ghusl (ritual bath), pray, and complete his fast."