Sahih Muslim Book 6, Hadith Number 2454.

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

Abu Bakr b. ‘Abd al-Rahman b. al-Harith b. Hisham reported on the authority of ‘Aisha and Umm Salama, the wives of the Apostle of Allah (may peace be upon him): The Messenger of Allah (may peace be upon him) at times got up in the morning in a state of junub on account of having a sexual intercourse (with his wives during night) but not due to sexual dreams in the month of Ramadan, and would observe fast.