It is narrated from Imam Muhammad bin Sirin, may Allah have mercy on him, regarding a man who has intercourse with his wife during her menstrual period, he said: He should seek forgiveness from Allah.
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1139
Hadith Takhrijاس قول کی سند صحیح ہے۔ دیکھئے: [مصنف عبدالرزاق 1267] و [ابن أبى شيبه 32/4/1]
Brief Explanation
(Commentary on Hadiths 1131 to 1139) All these narrations, despite being authentic in terms of chain of transmission, are athar (narrations from Companions) and mawquf statements (statements halted at the Companion), and in the authentic hadith, expiation (kaffarah) is also mentioned as will be discussed ahead. Therefore, if someone commits this mistake, he should repent and seek forgiveness, firmly resolve not to do so again in the future, and also give expiation (kaffarah).
Engaging in intercourse during menstruation (hayd) is not only a sin but also extremely harmful to health. Where Islam has taught inner purity and cleanliness, it has also prohibited apparent impurity and filth, and has taught to keep the outward self pure and clean as well.