Musannaf Ibn Abi Shayba 38:118

Ibn Idris narrated to us, 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 delivered a sermon to them on Friday and saw that they were few in number, so 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. Go forth, for whoever goes forth is safe. By Allah, we do not consider it well-being that these two armies meet, one striking the other; rather, we consider it well-being that Allah rectifies the nation of Muhammad and unites their harmony." "Shall I not tell you about Uthman and what the people held against him? They did not leave his sin until Allah Himself would punish him or pardon him. Those who sought him out, when they envied him for what Allah had given him, did not attain it." When Ali arrived, he said: "Are you the one who said what reached me about you, O Farruj? 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—you know that—and as for what remains of my intellect, we used to say that the latter [generation] after the latter is worse." He said: When he was at al-Silhin or al-Qadisiyyah, he went out to them while his two braids were dripping, appearing as though he had prepared for Ihram. When he placed his foot in the stirrup and took hold of the rear 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: "You must fear Allah and [adhere to] the community, for Allah will not gather the nation of Muhammad upon misguidance." They repeated it to him, and he said: "You must fear Allah and [adhere to] the community, for the righteous person finds rest, or [others] find rest from the wicked person."