كتاب التوبة
50  
The Book of Repentance
(8)
Chapter: The Acceptance Of The Repentance Of The One Who Kills, Even If He Has Killed A Great Deal
(8)
باب قَبُولِ تَوْبَةِ الْقَاتِلِ وَإِنْ كَثُرَ قَتْلُهُ ‏‏
Sahih Muslim 2766 b

Abu Sa'id al-Khudri reported Allah's Messenger (ﷺ) as saying that a man killed ninety-nine persons and then he began to make an inquiry whether there was any way left for him for repentance. He came to a monk and asked him about that, and he said:

There is no chance for repentance for you. He killed the monk also and then began to make an inquiry and moved from one village to another village where there lived pious persons, and as he had covered some distance, he was overtaken by death, but he managed to crawl upon his chest (to the side nearer to the place where the pious men lived). He died and then there was a dispute between the angels of mercy and the angels of punishment and (when it was measured) he was found to be nearer to the village where pious persons were living equal to the Space of a span and he was thus included among them.
حَدَّثَنِي عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ الْعَنْبَرِيُّ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، أَنَّهُ سَمِعَ أَبَا الصِّدِّيقِ النَّاجِيَّ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، عَنِ النَّبِيِّ صلى الله عليه وسلم ‏ "‏ أَنَّ رَجُلاً قَتَلَ تِسْعَةً وَتِسْعِينَ نَفْسًا فَجَعَلَ يَسْأَلُ هَلْ لَهُ مِنْ تَوْبَةٍ فَأَتَى رَاهِبًا فَسَأَلَهُ فَقَالَ لَيْسَتْ لَكَ تَوْبَةٌ ‏.‏ فَقَتَلَ الرَّاهِبَ ثُمَّ جَعَلَ يَسْأَلُ ثُمَّ خَرَجَ مِنْ قَرْيَةٍ إِلَى قَرْيَةٍ فِيهَا قَوْمٌ صَالِحُونَ فَلَمَّا كَانَ فِي بَعْضِ الطَّرِيقِ أَدْرَكَهُ الْمَوْتُ فَنَأَى بِصَدْرِهِ ثُمَّ مَاتَ فَاخْتَصَمَتْ فِيهِ مَلاَئِكَةُ الرَّحْمَةِ وَمَلاَئِكَةُ الْعَذَابِ فَكَانَ إِلَى الْقَرْيَةِ الصَّالِحَةِ أَقْرَبَ مِنْهَا بِشِبْرٍ فَجُعِلَ مِنْ أَهْلِهَا ‏"‏ ‏.‏
Reference : Sahih Muslim 2766b
In-book reference : Book 50, Hadith 55
USC-MSA web (English) reference : Book 37, Hadith 6663
  (deprecated numbering scheme)