It is narrated from Sayyidah Umm Salamah and Sayyidah Aishah (may Allah be pleased with them both) that dawn would come upon the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was in a state of major ritual impurity (janabah) from being with his wife and had not taken a ritual bath (ghusl), and then he (peace and blessings be upon him) would fast.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19897