Narrated 'Abdullah:
that the Prophet (ﷺ) said: "How horrible it is for one of them - or - one of you to say: "I have forgotten such and such Ayah,' rather he was made to forget. So be mindful of the Qur'an, for - by the One in Whose Hand is my soul - it escapes from men's hearts faster than a camel from its fetter." (DE)
English reference (DE): Vol. 5, Book 43, Hadith 2942 (DE)
Arabic reference (DE): Book 46, Hadith 3194 (DE)