Musnad Ahmad 3994

It was narrated from Ibn Mas'ood, who attributed it to the Prophet (ﷺ): "Keep revising the Qur'an, for by the One in whose hand is my soul, it is more likely to flee from the hearts of men than camels from their hobbles, What a bad thing for one of them to say, I forgot such and such a verse. Rather he was caused to forget."