It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) addressed us on the day of Ghadir and said: "Do I not have a greater right over the believers than they have over themselves?" The Companions said: Indeed, yes. Then he (peace and blessings be upon him) took the hand of Ali (may Allah be pleased with him) and said: "Whoever I am the mawla 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 / 5047