It is narrated from Sayyiduna Malik bin al-Huwayrith (may Allah be pleased with him); he said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever I am the mawla (patron) of, Ali (may Allah be pleased with him) is (also) his mawla."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16695