سنن ابن ماجه

It was narrated that Anas bin Malik said: “A man brought the killer of his relative to the Messenger of Allah (ﷺ) and the Messenger of Allah (ﷺ) said: 'Pardon him,' but the refused. He said: 'Take the blood money,' but he refused. He said: 'Go and kill him, but then you will be like him.’ Someone caught up with him and reminded him that the Messenger of Allah (ﷺ) had said: 'Go and kill him, but then you will be like him.’ So he let him go. He said: So he was seen dragging his strap going to his family. He said: It seemed that he had tied him up.

It's narrated that AbdurRahman bin AlQasim said "Then it is not (permissible) for anyone after the Prophet (ﷺ) ﷺ to say 'Go and kill him, but then you will be like him.' "