The Messenger of Allah ﷺ said, "A vow does not bring to the son of Adam anything I have not already decreed for him. Rather, the vow coincides with what I have already decreed. Through it, I extract from the miser, so he gives what he would not have given before."
حدثنا أبو داود قال قرئ على الحارث بن مسكين وأنا شاهد أخبركم ابن وهب قال أخبرني مالك عن أبي الزناد عن عبد الرحمن بن هرمز
أن رسول الله ﷺ قال لا يأتي ابن آدم النذر القدر بشيء لم أكن قدرته له ولكن يلقيه النذر القدر قدرته يستخرج من البخيل يؤتي عليه ما لم يكن يؤتي من قبل
Bukhari narrated it in "Oaths and Vows," in the chapter "Fulfilling the Vow," and Allah's saying, "They fulfill their vows" [Insan 76:7], hadith no. 6694, from the hadith of Abu Zinad. Muslim narrated it in "Vows," in the chapter "The Prohibition of Making Vows and That It Does Not Avert Anything," hadith no. 1640, from the hadith of Abdul-Rahman bin Hurmuz, through the same chain.