Narrated Ibn `Umar:

That a man came to him (while two groups of Muslims were fighting) and said, "O Abu `Abdur Rahman! Don't you hear what Allah has mentioned in His Book: 'And if two groups of believers fight against each other...' (49.9) So what prevents you from fighting as Allah has mentioned in His Book?"' Ibn `Umar said, "O son of my brother! I would rather be blamed for not fighting because of this Verse than to be blamed because of another Verse where Allah says: 'And whoever kills a believer intentionally..." (4.93) Then that man said, "Allah says:-- 'And fight them until there is no more afflictions (worshipping other besides Allah) and the religion (i.e. worship) will be all for Allah (Alone)" (8.39) Ibn `Umar said, "We did this during the lifetime of Allah's Messenger (ﷺ) when the number of Muslims was small, and a man was put to trial because of his religion, the pagans would either kill or chain him; but when the Muslims increased (and Islam spread), there was no persecution." When that man saw that Ibn `Umar did not agree to his proposal, he said, "What is your opinion regarding `Ali and `Uthman?" Ibn `Umar said, "What is my opinion regarding `Ali and `Uthman? As for `Uthman, Allah forgave him and you disliked to forgive him, and `Ali is the cousin and son-in-law of Allah's Messenger (ﷺ) ." Then he pointed out with his hand and said, "And that is his daughter's (house) which you can see."

It is narrated on the authority of Usama b. Zaid that the Messenger of Allah (ﷺ) sent us in a raiding party. We raided Huraqat of Juhaina in the morning. I caught hold of a man and he said: There is no god but Allah, I attacked him with a spear. It once occurred to me and I talked about it to the Apostle (ﷺ). The Messenger of Allah (ﷺ) said: Did he profess" There is no god but Allah," and even then you killed him? I said: Messenger of Allah, he made a profession of it out of the fear of the weapon. He (the Holy Prophet) observed: Did you tear his heart in order to find out whether it had professed or not? And he went on repeating it to me till I wished I had embraced Islam that day. Sa'd said: By Allah, I would never kill any Muslim so long as a person with a heavy belly, i. e., Usama, would not kill. Upon this a person remarked: Did Allah not say this: And fight them until there is no more mischief and religion is wholly for Allah? Sa'd said: We fought so that there should be no mischief, but you and your companions wish to fight so that there should be mischief.