Musnad Ahmad 12:8

'Ali ibn Ishaq narrated to us, 'Abdullah—meaning Ibn al-Mubarak—informed us, Zakariya ibn Ishaq informed us, from Ibn Abi Najih, that his father narrated to him, that he was informed by one who heard 'Abd al-Rahman ibn Abi Bakr saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Saddle this she-camel, then let your sister ride behind you. When you both descend from the hill of al-Tan'im, then enter into the state of Ihram and proceed." And that was on the night of departure.