Musnad Ahmad 1153:1003

'Affan narrated to us, he said: Abu 'Awanah narrated to us, from Mansur, from Ibrahim, from al-Aswad, from 'Aishah, she said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to command one of us, when she was menstruating, to wear an izar (a lower garment), then he would have physical contact with her."