Musnad Ahmad 32:809

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu al-Tayyah, that he heard Anas ibn Malik narrating that the Messenger of Allah (peace and blessings of Allah be upon him) said to Abu Dharr: "Listen and obey, even if it is an Abyssinian whose head is like a raisin."