What is the ruling on a husband having intercourse with his wife at the very beginning of her menstruation without the knowledge of either spouse?

If a man has intercourse with his wife and is unaware of the onset of menstruation, there is no sin upon them nor any expiation according to the majority of scholars, based on the Hadith (definition): "Indeed, Allah has forgiven my Ummah for their mistakes, forgetfulness, and that which they are forced to do." The Hanbalis, however, hold that expiation (a gold dinar) is obligatory even if he was unaware, and this is the more cautious view.