Musannaf Ibn Abi Shayba 7:242

12386. Humayd ibn Abd al-Rahman narrated to us, from Abu Bishr al-Jabali, from Abu Hurra, that Umar asked Ali, "What is your opinion regarding a man who has intercourse with his wife while she is menstruating?" He said, "There is no expiation upon him, except that he should repent."