كتاب الأيمان والنذور
Oaths and Vows
Chapter: To fulfil one's vow
باب الْوَفَاءِ بِالنَّذْرِ
Sahih al-Bukhari 6694

Narrated Abu Huraira:

The Prophet (ﷺ) said, "Allah says, 'The vow, does not bring about for the son of Adam anything I have not decreed for him, but his vow may coincide with what has been decided for him, and by this way I cause a miser to spend of his wealth. So he gives Me (spends in charity) for the fulfillment of what has been decreed for him what he would not give Me before but for his vow."

حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، حَدَّثَنَا أَبُو الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم ‏ "‏ لاَ يَأْتِي ابْنَ آدَمَ النَّذْرُ بِشَىْءٍ لَمْ يَكُنْ قُدِّرَ لَهُ، وَلَكِنْ يُلْقِيهِ النَّذْرُ إِلَى الْقَدَرِ قَدْ قُدِّرَ لَهُ، فَيَسْتَخْرِجُ اللَّهُ بِهِ مِنَ الْبَخِيلِ، فَيُؤْتِي عَلَيْهِ مَا لَمْ يَكُنْ يُؤْتِي عَلَيْهِ مِنْ قَبْلُ ‏"‏‏.‏
Reference : Sahih al-Bukhari 6694
In-book reference : Book 83, Hadith 71
USC-MSA web (English) reference : Vol. 8, Book 78, Hadith 685
  (deprecated numbering scheme)