It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to Ali (may Allah be pleased with him) on the Day of Ghadir Khumm: "O Allah, whoever I am his mawla, 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 / 5078