It is narrated from Amr bin Dhi Murr and Sayyiduna Zayd bin Arqam (may Allah be pleased with him); they said: The Messenger of Allah (peace and blessings be upon him) delivered a sermon on the day of Ghadir Khumm and said: "Whoever I am the mawla of, Ali is his mawla. O Allah! Befriend whoever befriends him, be hostile to whoever is hostile to him, help whoever helps him, and assist whoever assists him."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5040