Sahih Bukhari Volume 9, Book 93, Hadith Number 615.

Narrated By Musa bin Abi ‘Aisha : Sa’id bin Jubair reported from Ibn ‘Abbas (regarding the explanation of the Verse: ‘Do not move your tongue concerning (the Qur’an) to make haste therewith). He said, “The Prophet used to undergo great difficulty in receiving the Divine Inspiration and used to move his lips.’ Ibn ‘Abbas said (to Sa’id), “I move them (my lips) as Allah’s Apostle used to move his lips.” And Said said (to me), “I move my lips as I saw Ibn ‘Abbas moving his lips,” and then he moved his lips. So Allah revealed:

‘(O Muhammad!) Do not move your tongue concerning (the Qur’an) to make haste therewith. It is for Us to collect it and give you (O Muhammad) the ability to recite it. (i.e., to collect it in your chest and then you recite it).’ (75.16-17) But when We have recited it, to you (O Muhammad through Gabriel) then follow you its recital.’ (75.18) This means, “You should listen to it and keep quiet and then it is upon Us to make you recite it.”

The narrator added, “So Allah’s Apostle used to listen whenever Gabriel came to him, and when Gabriel left, the Prophet would recite the Qur’an as Gabriel had recited it to him.”