Sahih Muslim Book 4, Hadith Number 901.
Ibn Abbas reported with regard to the words: “Do not move thy tongue there with to make haste,” that the Apostle of Allah (may peace be upon him) felt it hard and he moved his lips. Ibn ‘Abbas said to me (Sa’id b. Jubair): I move them just as the Messenger of Allah (may peace be upon him) moved them. Then said Sa’id: I move them just as Ibn ‘Abbas moved them, and he moved his lips. Allah, the Exalted, revealed this: “Do not move your tongue therewith to make haste. It is with US that its collection rests and its recital” (Al-Qur’an, ixxv. 16). He said: Its preservation in your heart and then your recital. So when We recite it, follow its recital. He said: Listen to it, and be silent and then it rests with Us that you recite it. So when Gabriel came to the Messenger of Allah (may peace be upon him), he listened to him attentively, and when Gabriel went away, the Apostle of Allah (may peace be upon him) recited as he (Gabriel) had recited it.