Musnad Ahmad 1017:78

Ibn Numayr narrated to us, Ajlah al-Kindi narrated to me, from Abdullah ibn Buraydah, from his father Buraydah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, sent two expeditions to Yemen. Over one of them was Ali ibn Abi Talib, and over the other was Khalid ibn al-Walid. He said: "If you meet, then Ali is over the people, and if you separate, then each one of you is over his own troops." He said: We met the Banu Zayd of the people of Yemen and we fought. The Muslims prevailed over the polytheists, so we killed the combatants and took the offspring captive. Ali chose a woman from the captives for himself. Buraydah said: Khalid ibn al-Walid wrote with me to the Messenger of Allah, peace and blessings of Allah be upon him, informing him of that. When I came to the Prophet, peace and blessings of Allah be upon him, I handed him the letter, and it was read to him. I saw anger in the face of the Messenger of Allah, peace and blessings of Allah be upon him, so I said: "O Messenger of Allah, this is the place of the one seeking refuge. You sent me with a man and ordered me to obey him, so I did what I was sent for." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Do not disparage Ali, for he is from me and I am from him, and he is your guardian (wali) after me. And he is from me and I am from him, and he is your guardian (wali) after me."