A man from Basrah said: I heard Sayyiduna Imran bin Husayn (may Allah be pleased with him) say: I heard the Messenger of Allah (peace and blessings be upon him) say: "There is no vow in a matter of sin, and its expiation is equal to that of an oath."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15581