Sunan an-Nasa'i 4724
It was narrated that Wa'il said:
"I saw the Messenger of Allah when the heir of a victim brought the killer, leading him by a string. The Messenger of Allah said to the heir of the victim: Will you forgive him?' He said: 'No., He said: 'Will you accept Diyah?' He said: 'No.' He said: 'Will you kill him?' He said: 'Yes.' He said" 'Take him away (to kill him).' When he took him and turned away, he turned to those who were with him, and called him back, and said to him: 'Will you forgive him?' He said: No.' He said: 'Will you accept Diyah?' He said: No.' He said: 'Will you kill him?' He said: 'Yes.' He said: 'Take him away.' Then the Messenger of Allah said: 'If you forgive him, he will carry your sin and the sin of your companion (the victim).' So he forgave him and left him, and I was him dragging his string."
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ عَوْفِ بْنِ أَبِي جَمِيلَةَ، قَالَ حَدَّثَنِي حَمْزَةُ أَبُو عَمْرٍو الْعَائِذِيُّ، قَالَ حَدَّثَنَا عَلْقَمَةُ بْنُ وَائِلٍ، عَنْ وَائِلٍ، قَالَ شَهِدْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم حِينَ جِيءَ بِالْقَاتِلِ يَقُودُهُ وَلِيُّ الْمَقْتُولِ فِي نِسْعَةٍ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم لِوَلِيِّ الْمَقْتُولِ ‏"‏ أَتَعْفُو ‏"‏ ‏.‏ قَالَ لاَ ‏.‏ قَالَ ‏"‏ أَتَأْخُذُ الدِّيَةَ ‏"‏ ‏.‏ قَالَ لاَ ‏.‏ قَالَ ‏"‏ فَتَقْتُلُهُ ‏"‏ ‏.‏ قَالَ نَعَمْ ‏.‏ قَالَ ‏"‏ اذْهَبْ بِهِ ‏"‏ ‏.‏ فَلَمَّا ذَهَبَ بِهِ فَوَلَّى مِنْ عِنْدِهِ دَعَاهُ فَقَالَ لَهُ ‏"‏ أَتَعْفُو ‏"‏ ‏.‏ قَالَ لاَ ‏.‏ قَالَ ‏"‏ أَتَأْخُذُ الدِّيَةَ ‏"‏ ‏.‏ قَالَ لاَ ‏.‏ قَالَ ‏"‏ فَتَقْتُلُهُ ‏"‏ ‏.‏ قَالَ نَعَمْ ‏.‏ قَالَ ‏"‏ اذْهَبْ بِهِ ‏"‏ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم عِنْدَ ذَلِكَ ‏"‏ أَمَا إِنَّكَ إِنْ عَفَوْتَ عَنْهُ يَبُوءُ بِإِثْمِهِ وَإِثْمِ صَاحِبِكَ ‏"‏ ‏.‏ فَعَفَا عَنْهُ وَتَرَكَهُ فَأَنَا رَأَيْتُهُ يَجُرُّ نِسْعَتَهُ ‏.‏