Musnad Ahmad 1153:2295

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Yahya ibn Sa'id ibn Qays al-Ansari narrated to me, from 'Amrah bint 'Abd al-Rahman, from 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, who said: "When the Messenger of Allah, may Allah bless him and grant him peace, intended to go on a journey, he would cast lots among his wives. Whichever of them her lot came out for, he would take her with him."