Musnad Ahmad 1153:1392

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Mansur, from Ibrahim, from al-Aswad, from 'Aishah, that she said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to command one of us, if she was menstruating, to wrap an izar (lower garment) around herself, then he would lie with her." He said: "This is in al-Mubarak," then he would have physical contact with her.