What is the ruling on a husband having intercourse with his wife while she is menstruating, after she had believed she was pure?

If you had intercourse with your wife, thinking she was pure, and then blood appeared, there is no sin upon you. However, if intercourse occurred after the cessation of menstruation but before performing ghusl, then you must seek Allah's forgiveness, as intercourse is not permissible except after performing ghusl following purification.