Musannaf Ibn Abi Shayba 27:574

Ibn ‘Uyaynah narrated to us, from Dawud ibn Shabur, from Mujahid, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Observe my rights regarding al-‘Abbas, for he is the remainder of my forefathers, and a man’s paternal uncle is the counterpart of his father."