Ahmed bin Hafs bin 'Abdullah أحمد بن حفص بن عبد الله · The Book of Oaths and Vows

"I accompanied 'Imran bin Husain, who said: 'I heard the Messenger of Allah say: Vows are of two types: A vow that is made to do an act of obedience to Allah; that is for Allah and must be fulfilled, and a vow that is made to do an act of disobedience to Allah; that is for Shaitan and should not be fulfilled, and its expiation is the expiation for an oath.'"