Imam al-Sha'bi (may Allah have mercy on him) said: When Sayyiduna Ali (may Allah be pleased with him) set out for Siffin, he appointed Sayyiduna Abu Mas'ud (may Allah be pleased with him) as his deputy over Kufah. Some men from the people of Kufah had been hiding from Sayyiduna Ali (may Allah be pleased with him), and when he set out (for Siffin) they came out into the open. Some people would come to Sayyiduna Abu Mas'ud (may Allah be pleased with him) and say: "By Allah, Allah has destroyed his enemies and given victory to the believers." Abu Mas'ud (may Allah be pleased with him) would say: "By Allah, I do not consider it a victory or a well-being that one of the two groups prevails over the other." The people asked: "Then what is better?" He said: "That there be reconciliation between the people." When Sayyiduna Ali (may Allah be pleased with him) came back, the people mentioned this to him, and Sayyiduna Ali (may Allah be pleased with him) said: "Withdraw from our office." Abu Mas'ud (may Allah be pleased with him) said: "And why is that?" Sayyiduna Ali (may Allah be pleased with him) said: "We have found that your understanding has become somewhat weak." Abu Mas'ud (may Allah be pleased with him) said: "As for me, enough of my understanding remains for me to know that what comes later is worse than what came before."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14593