Musnad Abu Ya'la al-Mawsili 152:68

And with his chain of narration from the Messenger of Allah, may Allah bless him and grant him peace, he said: "It is a wretched thing for one of you to say, 'I forgot such and such a verse,' rather, he has been made to forget. Review the Quran, for it is more prone to escaping from the chests of men than camels from their hobbles."