Sahih Bukhari Volume 6, Book 60, Hadith Number 449.

Narrated By Ibn Abbas : The Prophet used to move his tongue when the divine Inspiration was being revealed to him. (Sufyan, a sub narrator, demonstrated (how the Prophet used to move his lips) and added. “In order to memorize it.” So Allah revealed: “Move not your tongue concerning (the Quran) to make haste therewith.” (75.16)