Musnad Ahmad 1153:817

Abu Nu'aym narrated to us, 'Abd al-Wahid ibn Ayman narrated to us, he said: Ibn Abi Mulaykah narrated to me, from al-Qasim, from 'Aishah, she said: "When the Messenger of Allah, peace and blessings of Allah be upon him, would go out, he would cast lots among his wives."