From Sahih Bukhari (Hadith 4928):
Narrated Musa bin Abi Aisha: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)
Reference: Bukhari 4928