Musnad Abu Ya'la al-Mawsili 217:15

Zuhayr narrated to us, Bishr ibn al-Sari narrated to us, Layth narrated to us, from al-Zuhri, from Habib, the freed slave of Urwah, from Nadbah, the freed slave of Maymunah, from Maymunah, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to have physical contact with his wives while they were menstruating, provided she was wearing an izar (waist wrapper) that reached the middle of her thighs or her knees, girding herself with it.