Narrated By 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 Apostle 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 Apostle 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 Qur’an) (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 (Qur’an) 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 Apostle used to listen to Gabriel whenever he came and after his departure he used to recite it as Gabriel had recited it.”
Sahih Bukhari Volume 001, Book 001, Hadith Number 004.