Sahih Muslim Book 6, Hadith Number 2452.

Chapter : There is no harm in observing fast if one is Junbi even after dawn.

‘Aisha, the wife of the Apostle of Allah (may peace be upon him), said: The dawn broke upon the Messenger of Allah (may peace be upon him) during the Ramadan in a state of junub not because of sexual dream (but on account of intercourse) and he washed himself and observed fast.