مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3935
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah that the Prophet (ﷺ) said: “What a bad thing for one of you - or for one of them - to say, I forgot such and such a verse. Rather he was caused to forget. Keep revising the Qur'an, for by the one іn Whose hand is my soul, it is more likely to flee from the hearts of men than camels from their hobbles."
Musnad Ahmad 3960Its isnad is saheeh, al-Bukhari (5032) and Muslim (790)]