Ibn Abbas (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Vows are of two kinds: 1. That which is for Allah, its expiation is fulfilling it. 2. That which is for Satan is not fulfilled; rather, the expiation of an oath must be given."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20078