Musnad Ahmad 26:410

Sulayman ibn Dawud narrated to us, Shu'bah narrated to us, from Mansur, who said: I heard Abu Wa'il narrating from Abdullah, from the Prophet (peace and blessings of Allah be upon him), who 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 an ayah,' rather, he was made to forget. Keep the Quran in remembrance, for by the One in Whose hand is my soul, it is more prone to escape from the chests of men than camels from their hobbles."