Musnad Ahmad 30:1613

Abu Salamah al-Khuza'i narrated to us, he said: Malik informed us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever swears an oath, then sees something else that is better than it, let him offer expiation for his oath, and let him do that which is better."