Ismail bin Abi Khalid and Amir Sha'bi, may Allah have mercy on them both, said regarding the person who has intercourse with his wife during her menstruation: He has committed a sin, he should seek forgiveness from Allah, repent, and not do it again in the future.
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1132