Musnad Abu Ya'la al-Mawsili 155:197

Abu Bakr ibn Abi Shaybah narrated to us, Ibn Fudayl narrated to us, from Ash'ath, from Ibn Sirin, who said: Abu Hurayrah said: The Messenger of Allah (ﷺ) said: "If ten of the Jews had followed me and believed in me, every Jew would have become a Muslim." Ka'b said: "Twelve." The confirmation of that is in [Surah] al-Ma'idah: "And Allah had taken a covenant from the Children of Israel, and We delegated from among them twelve leaders" (al-Ma'idah: 12).