Musnad Ahmad 1153:65

'Abd al-Rahman narrated to us, from Malik, from Sumayy and 'Abd Rabbih ibn Sa'id, from Abu Bakr ibn 'Abd al-Rahman, from 'A'ishah and Umm Salamah: "That the Messenger of Allah (ﷺ) would wake up in a state of janabah (major ritual impurity) from intercourse, not from a wet dream, and then he would fast." And she said in the hadith of 'Abd Rabbih: "In Ramadan."