Musnad Abu Ya'la al-Mawsili 216:12

Muhammad ibn Abd al-Rahman ibn Sahm al-Antaki narrated to us, Abdullah ibn al-Mubarak informed us, Ma'mar informed us, from al-Zuhri, from Nadbah, the freed slave woman of Maymunah, from Maymunah, who said: The Messenger of Allah (may Allah bless him and grant him peace) would have physical contact with a woman from his wives while she was menstruating, provided she had a cloth on her up to the middle of her thighs.