Sahih Muslim Book 6, Hadith Number 2456.
Chapter : There is no harm in observing fast if one is Junbi even after dawn.
Sulaiman b. Yasar reported that he asked Umm Salama whether a person (who gets up) in the morning in a state of junub should observe fast. She said: The Messenger of Allah (may peace be upon him) (at times) got up in the morning in a state of junub, not because of sexual dreams (but on account of intercourse at night), and then observed fast.