The Prophet (ﷺ) said, “Keep on reciting the Quran, for, by Him in Whose Hand my life is, Qur’an runs away (is forgotten) faster than camels that are released from their tying ropes.” (Bukhari)
This is the reason why you see “Da’ees” (people who call to Islam) and you are amazed and inspired by how much of the Quran and Hadith they can call recall from memory. They may have memorized these words like you and me, but because they spend their days and nights teaching people about Islam, they continue to read and recite and this helps them retain what they know.
