Musannaf Ibn Abi Shayba 7:84

Abu Bakr narrated to us, he said: Abd al-Rahim ibn Sulayman narrated to us, from Layth, from Mujahid, may Allah be pleased with him, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever swears by a Surah of the Qur'an, then upon him for every verse within it is a binding oath. So whoever wills, let him fulfill it, and whoever wills, let him break it."