Atiyyah al-Awfi narrates from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) came out at al-Juhfah on the day of Ghadir Khumm, holding the upper arm of Ali (may Allah be pleased with him), and said: "O people! Do you not know that I have a greater right over the believers than they have over themselves?" The Companions said: Indeed, yes, O Messenger of Allah! He said: "Then whoever I am the mawla of, this one (Ali) is his mawla."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5051