Musnad Ahmad 26:626

Muhammad ibn Ja‘far and Hajjaj narrated to us, they said: Shu‘bah narrated to us, from Mansur, from Abu Wa’il, from ‘Abdullah, from the Prophet, may Allah bless him and grant him peace, that he said: "It is a wretched thing for one of you—or a wretched thing for one of them—to say: 'I forgot such-and-such verse,' rather he was made to forget. And keep reviewing the Quran, for it is faster in escaping from the chests of men than camels from their tether, or from their tether."