Musnad Abu Ya'la al-Mawsili 6:82

Mahmud ibn Khidash narrated to us, Marwan ibn Mu'awiyah narrated to us, Qinan ibn Abdullah al-Nahmi narrated to us, Mus'ab ibn Sa'd ibn Abi Waqqas narrated to us, from his father, who said: I was sitting in the mosque, I and two men with me, and we spoke ill of Ali. Then the Messenger of Allah (peace and blessings of Allah be upon him) approached, angry, with the anger recognizable on his face. I sought refuge in Allah from his anger. He said: "What is the matter with you and me? Whoever harms Ali has indeed harmed me."