Sahih Muslim 1680
Sahih Muslim · Book 28: The Book of Oaths, Muharibin , Qasas (Retaliation), and Diyat (Blood Money) · A confession to murder is valid and the heir of the victim is entitled to Qisas, but it is recommended to ask him to let him go
'Alaqama b. Wa'il reported on the authority of his father that a person was brought to the Messenger of Allah (ﷺ) who had killed another person, and the heir of the person slain had dragged him (to the Holy Prophet) with a strap around his neck. As he turned away Allah's Messenger (ﷺ) said:
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ ، حَدَّثَنَا سَعِيدُ بْنُ سُلَيْمَانَ ، حَدَّثَنَا هُشَيْمٌ ، أَخْبَرَنَا إِسْمَاعِيلُ، بْنُ سَالِمٍ عَنْ عَلْقَمَةَ بْنِ وَائِلٍ ، عَنْ أَبِيهِ ، قَالَ أُتِيَ رَسُولُ اللَّهِ صلى الله عليه وسلم بِرَجُلٍ قَتَلَ رَجُلاً فَأَقَادَ وَلِيَّ الْمَقْتُولِ مِنْهُ فَانْطَلَقَ بِهِ وَفِي عُنُقِهِ نِسْعَةٌ يَجُرُّهَا فَلَمَّا أَدْبَرَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ " . فَأَتَى رَجُلٌ الرَّجُلَ فَقَالَ لَهُ مَقَالَةَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَخَلَّى عَنْهُ . قَالَ إِسْمَاعِيلُ بْنُ سَالِمٍ فَذَكَرْتُ ذَلِكَ لِحَبِيبِ بْنِ أَبِي ثَابِتٍ فَقَالَ حَدَّثَنِي ابْنُ أَشْوَعَ أَنَّ النَّبِيَّ صلى الله عليه وسلم إِنَّمَا سَأَلَهُ أَنْ يَعْفُوَ عَنْهُ فَأَبَى .
The killer and the killed are (doomed) to fire. A person came to the other person (the heir of the deceased) and he reported to him the words of the Messenger of Allah (ﷺ), and so he let him off. Isma'il b. Salim said: I made a mention of it to Habib b. Abu Thabit and he said: Ibn Ashwa' reported to me that Allah's Apostle (ﷺ) had asked him to pardon him, but he refused.