Muhammad bin al-Zubayr (may Allah have mercy on him) narrates from a companion of Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Vows are of two kinds: 1. A vow that is in obedience to Allah must be fulfilled. 2. A vow that is in disobedience to Allah is for Satan; it is not right to fulfil it, and its expiation is the expiation of an oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20071