Musnad Ahmad 30:2265

'Affan narrated to us, Abu Hilal narrated to us, he said: Muhammad ibn Sirin narrated to us, from Abu Hurairah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "If ten of the Jewish rabbis had believed in me, every Jew on the face of the earth would have believed in me." Ka'b said: "Twelve, their confirmation is in Surah Al-Ma'idah."