Musannaf Ibn Abi Shayba 27:446

'Abdullah ibn Numayr narrated to us, from al-'Ala' ibn al-Salih, from al-Minhal, from 'Abbad ibn 'Abdullah, who said: I heard 'Ali saying: "I am the servant of Allah and the brother of His Messenger, and I am the Greatest Truthful One (al-Siddiq al-Akbar). No one will say it after me except a liar and a fabricator. And I prayed before the people for seven years."