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." (75.16)
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 449
Arabic reference: Book 65, Hadith 4927