It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that she said: The Prophet (peace and blessings be upon him) would enter the morning in a state of major ritual impurity (janabah) without a wet dream, then he would take a ritual bath (ghusl) and continue his fast.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20223