Sahih Bukhari | Volume 8 | Hadith Number 685

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."