Abu Wa'il (may Allah have mercy on him) says that Sayyiduna Sahl bin Hunayf (may Allah be pleased with him) said on the day of Siffin: O people! Hold yourselves to blame. We were present with the Messenger of Allah (peace and blessings be upon him) at al-Hudaybiyyah. Had there been fighting, we would certainly have fought, but this was a peace treaty between the Muslims and the polytheists. At that time Sayyiduna Umar (may Allah be pleased with him) said: O Messenger of Allah (peace and blessings be upon him)! Are we not upon the truth and they upon falsehood? He (peace and blessings be upon him) said: "We are upon the truth and the enemy is upon falsehood." Sayyiduna Umar (may Allah be pleased with him) said: Will our slain not go to Paradise and their slain to Hell? He (peace and blessings be upon him) said: "Our slain are in Paradise and theirs are in Hell." Sayyiduna Umar (may Allah be pleased with him) said: Then why are we showing weakness, when it is Allah who will judge between us and them? He (peace and blessings be upon him) said: "O Ibn al-Khattab! I am the Messenger of Allah, and Allah will not abandon me." The narrator says that Sayyiduna Umar (may Allah be pleased with him), in a state of anger, went to Sayyiduna Abu Bakr (may Allah be pleased with him) and said: O Abu Bakr (may Allah be pleased with him)! Are we not upon the truth and they upon falsehood? Sayyiduna Abu Bakr (may Allah be pleased with him) said: We are upon the truth and they are upon falsehood. Sayyiduna Umar (may Allah be pleased with him) said: Will our slain not go to Paradise and theirs to Hell? Sayyiduna Abu Bakr (may Allah be pleased with him) said: Our slain are in Paradise and theirs are in Hell. Then Sayyiduna Umar (may Allah be pleased with him) said: Then why are we showing weakness in our religion, when Allah has not yet even judged between us and them? Sayyiduna Abu Bakr (may Allah be pleased with him) said: O Ibn al-Khattab! He is the Prophet of Allah, and Allah will not abandon him. The narrator says that Allah revealed the Qur'an, and he (peace and blessings be upon him) called Sayyiduna Umar (may Allah be pleased with him) and recited it to him. Sayyiduna Umar (may Allah be pleased with him) said: O Messenger of Allah (peace and blessings be upon him)! Is this a victory? He (peace and blessings be upon him) said: "This is a victory." The narrator says: Then he went back happily.
Imam al-Shafi'i (may Allah have mercy on him) says: Ibn Shihab (may Allah have mercy on him) said: There was no victory in Islam greater than this, when the fighting among the people came to an end. After that no sensible person heard the call of Islam without accepting it; in the two years of the truce more people became Muslim than had become Muslim before it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18813