What is the ruling on a husband having intercourse with his wife at the beginning of her menstruation out of their ignorance of the ruling, and does the ruling differ if the wife was not aware of the discharge of blood?

If her husband approaches her while she is menstruating and they are unaware of her menstruation, then there is no sin upon them, based on the saying of Allah the Almighty: {And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended. And ever is Allah Forgiving and Merciful.}