Sahih Muslim Book - 32, Hadith - 4634

وَحَدَّثَنَا أَبُو كُرَيْبٍ مُحَمَّدُ بْنُ الْعَلاَءِ وَمُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ قَالاَ حَدَّثَنَا أَبُو مُعَاوِيَةَ عَنِ الأَعْمَشِ عَنْ شَقِيقٍ قَالَ سَمِعْتُ سَهْلَ بْنَ حُنَيْفٍ يَقُولُ بِصِفِّينَ أَيُّهَا النَّاسُ اتَّهِمُوا رَأْيَكُمْ وَاللَّهِ لَقَدْ رَأَيْتُنِى يَوْمَ أَبِى جَنْدَلٍ وَلَوْ أَنِّى أَسْتَطِيعُ أَنْ أَرُدَّ أَمْرَ رَسُولِ اللَّهِ -صلى الله عليه وسلم- لَرَدَدْتُهُ وَاللَّهِ مَا وَضَعْنَا سُيُوفَنَا عَلَى عَوَاتِقِنَا إِلَى أَمْرٍ قَطُّ إِلاَّ أَسْهَلْنَ بِنَا إِلَى أَمْرٍ نَعْرِفُهُ إِلاَّ أَمْرَكُمْ هَذَا. لَمْ يَذْكُرِ ابْنُ نُمَيْرٍ إِلَى أَمْرٍ قَطُّ.

It was narrated that Shaqiq said: I heard Sahl bin Hunayf say at Siffin: O people, do not put too much faith in your own ideas, for by Allah, I remember the day of Abu Jandal (i.e. Al-Hudaibiyah). If I could have gone against the command of the Messenger of Allah (s.a.w) I would have done so. By Allah, we have never put our swords on our shoulders for any purpose, but the fighting resulted in a situation we feel comfortable with, except in this affair of yours (i.e., the fighting between 'Ali and Mu'awiyah, may Allah be pleased with them).