What must a husband do to repent to Allah after he had intercourse with his wife and then discovered that her menstrual period had begun?

If a man has intercourse with his wife while he is unaware that she is menstruating, there is no sin upon him, based on the saying of the Messenger of Allah (peace and blessings of Allah be upon him): "Indeed, Allah has forgiven my Ummah for their mistakes, forgetfulness, and what they are forced to do." However, the wife must inform her husband of her condition to avoid prohibited intercourse. If intercourse occurs without the knowledge of either party, there is no sin upon them.