What is the ruling on a husband having intercourse with his wife in Ramadan after the Fajr adhan, due to ignorance of the time?

If a fasting person in Ramadan engages in intercourse, believing it is still nighttime, then expiation (kaffarah) is not obligatory for him, but only making up the day (qada'). However, if he knew that dawn had broken, he has committed a grave sin, and the severe expiation (al-kaffarah al-mughallazah) becomes obligatory for him, along with making up the day.