Musannaf Ibn Abi Shayba 7:158

Muhammad ibn Bishr narrated to us, he said: Mis'ar narrated to us, he said: Ali ibn Zayd ibn Jud'an narrated to us, he said: Al-Hasan narrated to us, he said: Abd al-Rahman ibn Samura narrated to us, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If you swear an oath, then you see that which is better than it, then come to that which is better, and make expiation for your oath."