Musnad Ahmad 1017:11

Al-Fadl ibn Dukayn narrated to us, Ibn Abi Ghunyah narrated to us, from al-Hakam, from Sa’id ibn Jubayr, from Ibn Abbas, from Buraydah, who said: I went on a military expedition to Yemen with Ali, and I saw from him a certain coldness. When I arrived at the Messenger of Allah (peace and blessings of Allah be upon him), I mentioned Ali and spoke disparagingly of him. I saw the face of the Messenger of Allah (peace and blessings of Allah be upon him) change, and he said: "O Buraydah, am I not more worthy of the believers than their own selves?" I said: "Yes, O Messenger of Allah." He said: "Whoever I am his master (mawla), then Ali is his master (mawla)."