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)
Sahih al-Bukhari Book 65, Hadith 447
← Previous Hadith That he heard Allah's Messenger (ﷺ) describing the period of pause of the Divine Inspiration, and in his description he said, "While I was walking I heard a voice from the sky. I looked up towards the sky, and behold! I saw the same Angel who came to me in the Cave of Hira', sitting on a chair between the sky and the earth. I was so terrified by him that I fell down on the ground. Then I went to my wife and said, 'Wrap me in garments! Wrap me in garments!' They wrapped me, and then Allah revealed: "O you, (Muhammad) wrapped-up! Arise and warn...and desert the idols." (74.1-5) Abu Salama said....Rujz means idols." After that, the Divine Inspiration started coming more frequently and regularly.

Sahih al-Bukhari Book 65, Hadith 446 Next Hadith → That he asked Sa`id bin Jubair regarding (the statement of Allah). 'Move not your tongue concerning (the Qur'an) to make haste therewith.' He said, "Ibn `Abbas said that the Prophet (ﷺ) used to move his lips when the Divine Inspiration was being revealed to him. So the Prophet (ﷺ) was ordered not to move his tongue, which he used to do, lest some words should escape his memory. 'It is for Us to collect it' means, We will collect it in your chest;' and its recitation' means, We will make you recite it. 'But when We recite it (i.e. when it is revealed to you), follow its recital; it is for Us to explain it and make it clear,' (i.e. We will explain it through your tongue).

Sahih al-Bukhari Book 65, Hadith 448