Musannaf Ibn Abi Shayba 7:160

Waki’ narrated to us, from Hisham ibn ‘Urwah, from his father, from ‘Aishah, may Allah be pleased with her, who said: Abu Bakr would not swear an oath and then break it, until the expiation for oaths was revealed. Then he said: I will not swear an oath and then see another thing as better than it, except that I will do that which is better and offer expiation for my oath.