It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: I heard the Messenger of Allah (peace and blessings be upon him) say on the day of Ghadir Khumm: "Whoever I am the mawla (patron) of, Ali is his mawla. O Allah! Befriend whoever befriends him, and be hostile to whoever is hostile to him."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4964