Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.074C.

She may have intercourse with her husband even while the blood is flowing, according to most scholars, because there is no evidence to the contrary. Said Ibn ‘Abbas, “If she can pray, her husband can have intercourse with her.” Al-Bukhari says that if she is pure enough for prayer, she certainly must be pure enough for intercourse. Abu Dawud and al-Baihaqi related that ‘Akramah bint Hamnah had a prolonged flow of blood and that her husband had intercourse with her. An-Nawawi holds its chain to be hassan.