Musannaf Ibn Abi Shaybah 40681 — reports on Siffin

Kulayb al-Jarmi said: I was outside the mosque when I saw Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) returning from Sayyiduna Muawiyah (may Allah be pleased with him) in the matter of the two arbiters. He entered the house of Sulayman bin Rabi'ah, and I entered with him. One man taunted him, then another man taunted him, then yet another man taunted him, saying: "O Ibn Abbas (may Allah be pleased with them both), you have disbelieved, you have committed shirk, and you have set up an equal to Allah. Allah says this in His Book, and He says this, and He says this." This went on until something of it entered my own heart. The narrator was asked: "Who were they?" He said: "By Allah, they were of the first generation, companions of Muhammad (peace and blessings be upon him). By Allah, they were the people of the hooded cloaks and the pillars." When Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) heard what they said, he said: "Choose from among yourselves the most learned and the greatest debater, and let him speak to me." They chose a one-eyed man named Attab, from Banu Taghlib. He stood and said: "Allah Almighty says, Allah Almighty says," as though he were proving his need from a single surah of the Qur'an. On hearing him, Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "I consider you a scholar of the Qur'an, for you have presented your position very clearly. I ask you by the One besides whom there is no god: do you know that the people of Sham demanded the arbitration and we disliked it and refused? Then, when wounds reached you, pain reached you and you were deprived of the water of the Euphrates, you began to demand the arbitration. Sayyiduna Muawiyah (may Allah be pleased with him) told me that a thin-waisted horse was brought to him so that he might mount it and flee, until one of you came to him and said: 'I have left the people of Iraq like people running here and there on the night of departure in Makkah.'" Then Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "I ask you by the One besides whom there is no god: what kind of man was Abu Bakr?" All of them said he was a good man and praised him. Then he asked: "What kind of man was Umar bin al-Khattab?" All of them said he was a good man and praised him. Then Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "What is your view: if a man set out for Hajj or Umrah and killed a deer or one of the small creatures of that place, and one of those two judged the case alone, would he have been entitled to that, when Allah Almighty says: 'Two just men shall judge it' [Al-Ma'idah: 95]? And what you differ over is far greater than this. When Allah Almighty has appointed two arbiters for justice and fairness in the matter of a bird, and two arbiters in the matter of a man and his wife, why can there not be two arbiters in your dispute, which is greater than these?"