سنن النسائي

It was narrated from 'Abdullah bin Buraidah, from the father, that a man came to the Prophet (ﷺ) and said: "This man killed my brother." He said: "Go and kill him as he killed your brother." The man said to him: "Fear Allah and let me go, for that will bring you a greater reward and will be better for you and your brother on the Day of Resurrection." So he let him go. The Prophet (ﷺ) was told about that, so he asked him about it, and he told him what he had said. He said: "Pardoning him would be better for you than what he would have done for you on the Day of Resurrection when he said: 'O Lord, ask him why he killed me."'