Musnad Abu Ya'la al-Mawsili 154:132

Muhammad ibn Yahya al-Zamani narrated to us, Muhammad ibn al-Harith al-Harithi narrated to us, Muhammad ibn Abd al-Rahman al-Baylamani narrated to us, from his father, from Ibn Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever swears an oath, then sees other than it—meaning better than it—then its expiation is leaving it."