Musannaf Ibn Abi Shayba 9:931

Abu al-Ahwas narrated to us, from Tariq, from Asim ibn Amr al-Bajali, who said: Some people from the people of Iraq went out, and when they arrived to Umar, he said to them: "Who are you?" They said: "From the people of Iraq." He said: "And did you come with permission?" They said: "Yes." Then they asked about what is lawful for a man from his wife while she is menstruating. He said: "You have asked me about matters that no one has asked me about since I asked the Messenger of Allah (peace and blessings of Allah be upon him)." He said: "As for what a man has from his wife while she is menstruating, he has what is above the waist-wrapper."