كتاب التوبة
50
The Book of Repentance
(4)
Chapter: The Vastaess Of Allah's Mercy, Which Prevails Over His Wrath
(4)
باب فِي سَعَةِ رَحْمَةِ اللَّهِ تَعَالَى وَأَنَّهَا سَبَقَتْ غَضَبَهُ ‏‏

Salman Farisi reported Allah's Messenger (ﷺ) as saying:

Verily, there are one hundred (parts of) mercy for Allah, and it is one part of this mercy by virtue of which there is mutual love between the people and ninety-nine reserved for the Day of Resurrection.
حَدَّثَنِي الْحَكَمُ بْنُ مُوسَى، حَدَّثَنَا مُعَاذُ بْنُ مُعَاذٍ، حَدَّثَنَا سُلَيْمَانُ التَّيْمِيُّ، حَدَّثَنَا أَبُو عُثْمَانَ النَّهْدِيُّ عَنْ سَلْمَانَ الْفَارِسِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِنَّ لِلَّهِ مِائَةَ رَحْمَةٍ فَمِنْهَا رَحْمَةٌ بِهَا يَتَرَاحَمُ الْخَلْقُ بَيْنَهُمْ وَتِسْعَةٌ وَتِسْعُونَ لِيَوْمِ الْقِيَامَةِ ‏"‏ ‏.‏
Reference : Sahih Muslim 2753 a
In-book reference : Book 50, Hadith 23
USC-MSA web (English) reference : Book 37, Hadith 6632
  (deprecated numbering scheme)