Ibn 'Abbas reported with regard to the words of Allah, Great and Glorious:

Move not thy tongue therewith (Ixxv. 16) that when Gabriel brought revelation to him (the Holy Prophet) he moved his tongue and lips (with a view to committing it to memory instantly). This was something hard for him and it was visible (from his face). Then Allah, the Exalted. revealed this a Move not thy tongue therewith to make haste (in memorising it). Surely on us rests the collecting of it and the reciting of it (ixxv. 16), i. e. Verily it rests with Us that We would preserve it in your heart and (enable you) to recite it You would recite it when We would recite it and so follow its recitation, and He (Allah) said: We revealed it, so listen to it attentively. Verily its exposition rests with Us. i. e. We would make it deliver by your tongue. So when Gabriel came to him (to the Holy Prophet), he kept silence, and when he went away he recited as Allah had promised him.