Musannaf Ibn Abi Shayba 37:562

Abdullah 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 addressed them on a Friday, and seeing their scarcity, he said: "O people, go forth! Whoever goes forth is safe. By Allah, we know that among you are those who dislike this expedition and are sluggish regarding it, so go forth, for whoever goes forth is safe. By Allah, we do not consider it 'well-being' for these two groups to meet, with each one fearing the other. Rather, we consider it 'well-being' for Allah to rectify the nation of Muhammad and unite their harmony. Shall I not tell you about Uthman and what the people held against him? They will not leave him and his sin until Allah is the One who punishes him or pardons him. They did not attain what they sought when they envied him for what Allah had given him." When Ali arrived, he said to him: "Are you the one who said what reached me about you, O 'Farruj' (chicken/coward)? You are an old man whose intellect has departed." He replied: "My mother named me with a name better than this. My intellect has departed, yet Paradise has become incumbent upon me from Allah and His Messenger, which you know. As for what remains of my intellect, we used to say that the latter and the latter is worse." Then he departed. When he was at al-Silhin or al-Qadisiyyah, he went out to them while his two braids were dripping [with water], and they thought he had prepared for Ihram. When he placed his foot in the stirrup and took hold of the back of the saddle, some people stood up to him and said: "If only you would leave us a testament, O Abu Mas’ud." He said: "Fear Allah and [adhere to] the community (al-Jama'ah), for Allah will not gather the nation of Muhammad upon misguidance." They repeated it to him, and he said: "Upon you is the fear of Allah and the community, for the righteous person finds rest, or [the people] find rest from the wicked person."