What is the ruling on a husband having intercourse with his wife while she is menstruating, when ignorant of the ruling?

Ignorance of a religious ruling is an excuse before Allah. Scholars have stated that whoever commits a forbidden act out of ignorance bears no sin and owes no expiation. Allah has shown mercy to this Ummah by removing sin from them in cases of ignorance, forgetfulness, and coercion. The proof for this is Allah’s saying: "Our Lord, do not take us to task if we forget or make a mistake," and the Prophet’s (peace be upon him) saying: "I have done so." Therefore, whoever has intercourse with his menstruating wife out of ignorance of its prohibition bears no sin and owes no expiation. However, it is incumbent upon every Muslim to strive to gain understanding in the religion and learn its rulings.