Musannaf Ibn Abi Shayba 27:664

Ibn Idris narrated to us, from Layth, from Abd al-Aziz ibn Rufay', who said: When Ali marched to Siffin, he appointed Abu Mas'ud as a deputy over the people. He said: When Ali arrived, he said to him, "Are you the one who said what has reached me about you, O Farrukh? You are an old man whose intellect has departed." He said, "Has my intellect departed, while you have made Paradise incumbent upon me regarding Allah and His Messenger? You know it."