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 Apostle 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 Apostle ." Then he pointed out with his hand and said, "And that is his daughter's (house) which you can see."
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 173
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 173