Musnad Ahmad 28:512

Abdullah ibn Bakr narrated to us, Ubaydullah ibn al-Akhnas Abu Malik al-Azdi narrated to us, from Amr ibn Shu'ayb, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no vow and no oath in what the son of Adam does not possess, nor in disobedience to Allah the Almighty, nor in severing ties of kinship. Whoever swears an oath and then sees that something else is better than it, let him leave it and do that which is better, for leaving it is its expiation."