Musnad Ahmad 26:865

Affan narrated to us, Hammad bin Zayd narrated to us, Asim bin Bahdalah narrated to us, and Mansur bin al-Mu'tamir narrated to us, from Abu Wa'il, from Abdullah, who said: The Messenger of Allah (ﷺ) said: "It is a wretched thing for one of them—or one of you—to say: 'I forgot such-and-such verse,' rather, he was made to forget. So, refresh your memory of the Quran, for it is more prone to escaping from the chests of men than camels are from their tethering ropes." He said: Or he said: "From its tethering rope."