Abdullah bin Murrah narrates from Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) forbade vows and said: "A vow does not avert anything; it is only a means by which wealth is extracted from the miser."
(b) In the narration of Khallad: "Wealth is extracted by it from the miser."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20103