Musnad Ahmad 1153:815

Abu Nu'aym narrated to us, he said: 'Abd al-Malik ibn Humayd ibn Abi Ghaniyah narrated to us, from Thabit ibn 'Ubayd, from al-Qasim, from 'Aishah, she said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Hand me the prayer mat from the mosque." I said: "I am menstruating." He said: "Indeed, your menstruation is not in your hand."