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.