Narrated Ibn Abbas:
(as regards) Allah's Statement: "Move not your tongue concerning (the Quran) to make haste therewith." (75.16) When Gabriel revealed the Divine Inspiration in Allah's Apostle , he (Allah's Apostle) moved his tongue and lips, and that state used to be very hard for him, and that movement indicated that revelation was taking place. So Allah revealed in Surat Al-Qiyama which begins: 'I do swear by the Day of Resurrection...' (75) the Verses:-- 'Move not your tongue concerning (the Quran) to make haste therewith. It is for Us to collect it (Quran) in your mind, and give you the ability to recite it by heart. (75.16-17) Ibn Abbas added: It is for Us to collect it (Qur'an) (in your mind), and give you the ability to recite it by heart means, "When We reveal it, listen. Then it is for Us to explain it," means, 'It is for us to explain it through your tongue.' So whenever Gabriel came to Allah's Apostle ' he would keep quiet (and listen), and when the Angel left, the Prophet would recite that revelation as Allah promised him.
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 451
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 451