Riyah bin al-Harith (may Allah have mercy on him) narrates: Sayyiduna Ali (may Allah be pleased with him) was sitting in al-Rahbah (the open square of Kufah) when a man came bearing the traces of travel. He said: Peace be upon you, O my mawla! It was asked: Who is this? It was said: Abu Ayyub al-Ansari (may Allah be pleased with him). (Sayyiduna Ali (may Allah be pleased with him)) said: O Abu Ayyub, did you hear the Messenger of Allah (peace and blessings be upon him) say: "Whoever I am the mawla of, Ali is his mawla"?
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4037