Musnad Ahmad 1163:55

Hajjaj and Abu Kamil narrated to us, they said: Layth ibn Sa’d narrated to us, he said: Ibn Shihab narrated to me, from Habib, the freed slave of ‘Urwah, from Nadbah, the freed slave of Maymunah, from Maymunah, the wife of the Prophet (peace and blessings of Allah be upon him): That the Messenger of Allah (peace and blessings of Allah be upon him) used to have physical contact with a woman from his wives while she was menstruating, if she had on a lower garment (izar) that reached the middle of her thighs or her knees, tucked in with it.