Sahih Bukhari
Oaths and Vows
Volume: 008 / Book Number: 078 / Hadith Number: 683
Narrated Sa'id bin Al-Harith:
that he heard Ibn 'Umar saying, "Weren't people forbidden to make vows?" The Prophet said, 'A vow neither hastens nor delays anything, but by the making of vows, some of the wealth of a miser is taken out."