Musnad Ahmad 1163:58

'Abd al-Razzaq narrated to us, he said: Ma'mar narrated to us, from al-Zuhri, from Nudbah, the freed slave woman of Maymunah, from Maymunah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to have physical contact with his wives while they were menstruating, provided they had a cloth on up to the knees or up to the middle of the thighs."