Abu al-Tufayl narrates from Zayd bin Arqam or Hudhayfah bin Asid (may Allah be pleased with them both) (the narrator was unsure) that the Prophet (peace and blessings be upon him) said: "Whoever I am the mawla (patron, beloved friend) of, Ali is his mawla."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3036