Musnad Ahmad 8:169

Qutaybah ibn Sa'id narrated to us, Hatim ibn Isma'il narrated to us, from Bukayr ibn Mismar, from 'Amir ibn Sa'd, from his father, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say to him [Ali] when he left him behind in one of his military expeditions. Ali said: "O Messenger of Allah, do you leave me behind with the women and the children?" He said: "O Ali, are you not pleased that you should be to me in the position of Aaron to Moses, except that there is no prophethood after me?" And I heard him say on the day of Khaybar: "I shall certainly give the banner to a man who loves Allah and His Messenger, and whom Allah and His Messenger love." We stretched ourselves out for it, then he said: "Call Ali for me." He was brought, suffering from an eye ailment, so he spat into his eye and handed the banner to him, and Allah granted victory through him. And when this verse was revealed: {Let us call our sons and your sons} [Al 'Imran: 61], the Messenger of Allah, peace and blessings of Allah be upon him, called Ali, Fatimah, Hasan, and Husayn, may the pleasure of Allah be upon them, and said: "O Allah, these are my family."