Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 449
Narrated Ibn Abbas:
The Prophet used to move his tongue when the divine Inspiration was being revealed to him. (Sufyan, a subnarrator, 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)