Sayyidah Aishah (may Allah be pleased with her) says that dawn would come upon the Messenger of Allah (peace and blessings be upon him) in Ramadan while he (peace and blessings be upon him) was in a state of janabah, not from a wet dream; then he (peace and blessings be upon him) would take a ritual bath and fast.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7994