:
“When the Messenger of Allah died and Abu Bakr was appointed as Khalifah, and some of the Arab’s disbelieved, Umar said: ‘O Abu Bakr! How can you fight the people when the Messenger of Allah said: I have been commanded to fight the people until they say La ilaha illallah (there is none worthy of worship except Allah). Whoever says La ilaha illallah, his life and his property are safe from me, except for its right, and his reckoning will be with Allah?’ Abu Bakr, may Allah be pleased with him, said: ‘By Allah, I will surely fight those who separate prayer and Zakah, for Zakah is what is due on wealth. By Allah, if they withhold from me a small she-goat that they used to give to the Messenger of Allah I will fight them for withholding it.’ (Umar said) ‘By Allah, when I realized that Allah, the Mighty and Sublime, had opened the chest of Abu Bakr to fighting, then I knew that it was the truth.’”
Sunan an-Nasa'i Book 25, Hadith 7
I went to Abu Wail to ask him (about those who had rebelled against `Ali). On that Abu Wail said,
"We were at Siffin (a city on the bank of the Euphrates, the place where me battle took place between
`Ali and Muawiya) A man said, "Will you be on the side of those who are called to consult Allah's
Book (to settle the dispute)?" `Ali said, 'Yes (I agree that we should settle the matter in the light of the
Quran)." ' Some people objected to `Ali's agreement and wanted to fight. On that Sahl bin Hunaif
said, 'Blame yourselves! I remember how, on the day of Al-Hudaibiya (i.e. the peace treaty between
the Prophet (ﷺ) and the Quraish pagans), if we had been allowed to choose fighting, we would have
fought (the pagans). At that time `Umar came (to the Prophet) and said, "Aren't we on the right (path)
and they (pagans) in the wrong? Won't our killed persons go to Paradise, and theirs in the Fire?" The
Prophet replied, "Yes." `Umar further said, "Then why should we let our religion be degraded and
return before Allah has settled the matter between us?" The Prophet (ﷺ) said, "O the son of Al-Khattab!
No doubt, I am Allah's Messenger (ﷺ) and Allah will never neglect me." So `Umar left the place angrily and
he was so impatient that he went to Abu Bakr and said, "O Abu Bakr! Aren't we on the right (path)
and they (pagans) on the wrong?" Abu Bakr said, "O the son of Al-Khattab! He is Allah's Messenger (ﷺ), and
Allah will never neglect him." Then Sura Al-Fath (The Victory) was revealed."
Sahih al-Bukhari Book 65, Hadith 365