Narrated Said bin Jubair:
Ibn 'Abbas in the explanation of the statement of Allah "Move not your tongue concerning (the Quran) to make haste therewith." (75.16) said "Allah's Messenger (ﷺ) used to bear the revelation with great trouble and used to move his lips (quickly) with the Inspiration." Ibn 'Abbas moved his lips saying, "I am moving my lips in front of you as Allah's Messenger (ﷺ) used to move his." Said moved his lips saying: "I am moving my lips, as I saw Ibn 'Abbas moving his." Ibn 'Abbas added, "So Allah revealed 'Move not your tongue concerning (the Qur'an) to make haste therewith. It is for Us to collect it and to give you (O Muhammad) the ability to recite it (the Quran)' (75.16-17) which means that Allah will make him (the Prophet) remember the portion of the Qur'an which was revealed at that time by heart and recite it. The statement of Allah: 'And when we have recited it to you (O Muhammad through Gabriel) then you follow its (Quran) recital' (75.18) means 'listen to it and be silent.' Then it is for Us (Allah) to make it clear to you' (75.19) means 'Then it is (for Allah) to make you recite it (and its meaning will be clear by itself through your tongue). Afterwards, Allah's Messenger (ﷺ) used to listen to Gabriel whenever he came and after his departure he used to recite it as Gabriel had recited it."
Narrated Ibn `Abbas:
Regarding His (Allah's) Statement:-- 'Move not your tongue concerning (the Qur'an) to make haste therewith.' (75.16) And whenever Gabriel descended to Allah's Messenger (ﷺ) with the Divine Inspiration, Allah's Messenger (ﷺ) used to move his tongue and lips, and that used to be hard for him, and one could easily recognize that he was being inspired Divinely. So Allah revealed the Verse which occurs in the Surah starting with "I do swear by the Day of Resurrection.' (75.1) i.e. 'Move not your tongue concerning (the Qur'an) to make haste then with. It is for Us to collect it (in your mind) (O Muhammad) an give you the ability to recite it 'by heart.' (75.16-17) which means: It is for us to collect it (in your mind) and give you the ability to recite it by heart. And when We have recited it to you (O Muhammad) through Gabriel then follow you its recital. (75.18) means: 'When We reveal it (the Qur'an) to you, Listen to it.' for then: It is for Us to explain it and make it clear to you' (75.19) i.e. It is up to Us to explain it through your tongue. So, when Gabriel came to him, Allah's Messenger (ﷺ) would listen to him attentively, and as soon as Gabriel left, he would recite the Revelations, as Allah had promised him.