It is narrated from Sayyiduna Amr bin Shurahil (may Allah be pleased with him); he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "O Allah, help Ali. O Allah, honour whoever honours Ali. O Allah, forsake whoever forsakes Ali (may Allah be pleased with him)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14154