The Prophet ﷺ said, "How wretched it is for one of you to say, 'I have forgotten such-and-such a verse.' Rather, he was made to forget. So keep reviewing the Quran, for it slips away from the hearts of Some men faster than livestock from their tethers."
قال النبي ﷺ: بئس ما لأحدهم أن يقول: نسيت آية كيت وكيت، بل نسي، و استذكروا القرآن، فإنه أشد تفصيا من صدور الرجال من النعم.
Bukhari related it in the Virtues of the Quran (5032) and Muslim in the Prayer of the Traveler (790), both from the hadith of Mansur, from Abu Waail, from Abdullah, as narrated above. The wording given is that of Bukhari, and Muslim's wording is the same, but he added, after the words "than a camel," the phrase "from its tether."In another version, related by Muslim from the hadith of Amash, from Shaqiq (who is Abu Waail), he said, Abdullah said, "Keep up your recitation of these copies of the Quran," and sometimes he said "the Quran," "for it slips away from the hearts of men faster than a camel from its tether." He also said, the Messenger of Allah ﷺ said, "None of you should say, 'I have forgotten such and such a verse,' rather he has been made to forget it."The word "tafassiyan" means slipping away or escaping. Tafassi means separation or detachment.