Musnad Ahmad 5:645

Muhammad ibn Bishr narrated to us, Hisham ibn Urwah narrated to us, from his father, that Abdullah ibn Ja'far narrated to him that he heard Ali saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The best of its women is Maryam bint Imran, and the best of its women is Khadijah."