From Sahih Bukhari (Hadith 4927):
Narrated Ibn `Abbas:The Prophet (ﷺ) used to move his tongue when the divine Inspiration was being revealed to him. (Sufyan, a subnarrator, demonstrated (how the Prophet (ﷺ) used to move his lips) and added. "In order to memorize it." So Allah revealed: "Move not your tongue concerning (the Qur'an) to make haste therewith
Reference: Bukhari 4927