Atiyyah narrates from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) 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." I asked him for more, and he said: I only go as far as I was taken.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5050