Musnad Ahmad 28:260

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Khalifah ibn Khayyat narrated to us, ‘Amr ibn Shu’ayb narrated to me, from his father, from his grandfather, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever swears an oath, then sees something else better than it, let him leave it, and its expiation is [to do] that."