Musnad Ahmad 28:491

'Abd al-Samad narrated to us, Khalifah ibn Khayyat al-Laythi narrated to us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever swears an oath, then sees something else to be better than it, then that is its expiation."