Abdullah bin Abbas reported,

Whenever Jibreel came down to the Messenger of Allah ﷺ with revelation, he would move his tongue and lips along with it, and this weighed heavily upon him, and it could be noticed in him. So Allah revealed the verse in the surah that begins "I swear by the Day of Resurrection" [Qiyamah 75:1], namely "Do not move your tongue with it to hasten it. Upon Us is its collection and its recitation" [Qiyamah 75:16–17].Ibn Abbas explained, "It is upon Us to gather it in your breast." As for "and its recitation, so when We recite it, follow its recitation" [Qiyamah 75:17–18], this means, when We send it down, listen. And "Then upon Us is its clarification" [Qiyamah 75:19] means, it is upon Us to make it clear through your tongue.He said, so after that, whenever Jibreel came to him, he would listen in silence, and once Jibreel departed he would recite it just as Allah had promised him.

في قوله: {لا تحرك به لسانك لتعجل به}

عن ابن عباس قال

كان رسول الله ﷺ إذا نزل جبريل بالوحي، وكان مما يحرك به لسانه وشفتيه فيشتد عليه، وكان يعرف منه، فأنزل الله الآية التي في{لا أقسم بيوم القيمة} …: {لا تحرك به لسانك لتعجل به إن علينا جمعه وقرانه} قال: علينا أن نجمعه في صدرك، {وقرءانه فإذا فاتبع قرآنه} فإذا أنزلناه فاستمع، {ثم إن علينا بيانه}، علينا أن نبينه بلسانك قال: فكان إذا أتاه جبريل أطرق فإذا ذهب قرأه كما وعده الله.