Sahih Bukhari Volume 8, Book 83, Hadith 6696

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 ‏"

Narrated By Abu Huraira : The Prophet 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 fulfilment of what has been decreed for him what he would not give Me before but for his vow."