Musnad Ahmad 1153:423

Musa ibn Dawud narrated to us, Mubarak narrated to us, from Abu Imran al-Jawni, from Yazid ibn Babnus, from Aisha, from the Prophet, peace and blessings of Allah be upon him, regarding a man who has intimate contact with his wife while she is menstruating. He said to him: "What is above the waist-wrapper."