Musnad Abu Ya'la al-Mawsili 3:81

Ubayd Allah narrated to us, Waki' narrated to us, from al-A'mash, from Ibn Abi al-Ja'd, from Abdullah ibn Saba' who said: It was said to Ali, "Will you not appoint a successor?" He said, "No, but I will leave you to what the Messenger of Allah, may Allah bless him and grant him peace, left you."