It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said on the Day of Ghadir Khumm: "Whoever I am his mawla, Ali is his mawla."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5077