Musannaf Ibn Abi Shayba 7:153

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Mughirah, from Ibrahim, who said: If it is said to a man, "You swore that you would not do such and such," and he says, "Yes," while he had not sworn, he said: "Upon him is the expiation of an oath."