It is narrated from Sa'id bin Wahb, Habbah al-Urani and Sayyiduna Zayd bin Arqam (may Allah be pleased with him) that Sayyiduna Ali (may Allah be pleased with him) adjured the people by Allah (asking) that whoever had heard the Messenger of Allah (peace and blessings be upon him) saying: "Whoever I am the wali (ally) of, Ali is his wali," should bear witness. So ten-odd men stood up and testified that they had heard the Messenger of Allah (peace and blessings be upon him) saying: "Whoever I am the wali of, Ali is his wali."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5039