Musnad Ahmad 25:1332

'Abd al-Rahman narrated to us, from Abu 'Awanah, from Musa ibn Abi 'Aishah, from Sa'id ibn Jubayr, from Ibn 'Abbas, regarding His saying: "Do not move your tongue with it to hasten it" [Al-Qiyamah: 16]. He said: "The Prophet, may Allah bless him and grant him peace, used to struggle with the revelation, and he would move his lips." He said: Ibn 'Abbas said to me, "I am moving my lips just as the Messenger of Allah, may Allah bless him and grant him peace, used to move them." And Sa'id said to me, "I am moving them just as I saw Ibn 'Abbas moving his lips." Then Allah, the Mighty and Majestic, revealed: "Do not move your tongue with it to hasten it. Indeed, upon Us is its collection and its recitation" [Al-Qiyamah: 16-17]. He said: "Its collection in your chest, then we recite it." "So when We have recited it, follow its recitation" [Al-Qiyamah: 18]. He said: "Listen to it and remain silent." "Then, indeed, upon Us is its clarification" [Al-Qiyamah: 19]. He said: "After that, when Gabriel would depart, he would recite it just as he had recited it to him."