The Prophet ﷺ used to move his tongue when the divine Inspiration was being revealed to him. (Sufyan, a subnarrator, demonstrated (how the Prophet ﷺ used to move his lips) and added. "In order to memorize it." So Allah revealed: "Move not your tongue concerning (the Qur'an) to make haste therewith." (75.16)  

كَانَ النَّبِيُّ ﷺ إِذَا نَزَلَ عَلَيْهِ الْوَحْىُ حَرَّكَ بِهِ لِسَانَهُ وَوَصَفَ سُفْيَانُ يُرِيدُ أَنْ يَحْفَظَهُ فَأَنْزَلَ اللَّهُ {لاَ تُحَرِّكْ بِهِ لِسَانَكَ لِتَعْجَلَ بِهِ}  

That he asked Saʿid bin Jubair regarding (the statement of Allah). 'Move not your tongue concerning (the Qur'an) to make haste therewith.' He said, "Ibn ʿAbbas said that the Prophet ﷺ used to move his lips when the Divine Inspiration was being revealed to him. So the Prophet ﷺ was ordered not to move his tongue, which he used to do, lest some words should escape his memory. 'It is for Us to collect it' means, We will collect it in your chest;' and its recitation' means, We will make you recite it. 'But when We recite it (i.e. when it is revealed to you), follow its recital; it is for Us to explain it and make it clear,' (i.e. We will explain it through your tongue).  

عَنْ قَوْلِهِ تَعَالَى {لاَ تُحَرِّكْ بِهِ لِسَانَكَ} قَالَ وَقَالَ ابْنُ عَبَّاسٍ كَانَ يُحَرِّكُ شَفَتَيْهِ إِذَا أُنْزِلَ عَلَيْهِ فَقِيلَ لَهُ {لاَ تُحَرِّكْ بِهِ لِسَانَكَ} يَخْشَى أَنْ يَنْفَلِتَ مِنْهُ {إِنَّ عَلَيْنَا جَمْعَهُ وَقُرْآنَهُ} أَنْ نَجْمَعَهُ فِي صَدْرِكَ وَقُرْآنَهُ أَنْ تَقْرَأَهُ {فَإِذَا قَرَأْنَاهُ} يَقُولُ أُنْزِلَ عَلَيْهِ {فَاتَّبِعْ قُرْآنَهُ * ثُمَّ إِنَّ عَلَيْنَا بَيَانَهُ} أَنْ نُبَيِّنَهُ عَلَى لِسَانِكَ  

(as regards) Allah's Statement: "Move not your tongue concerning (the Qur'an) to make haste therewith." (75.16) When Gabriel revealed the Divine Inspiration in Messenger of Allah ﷺ , he (Messenger of Allah ﷺ) moved his tongue and lips, and that state used to be very hard for him, and that movement indicated that revelation was taking place. So Allah revealed in Surat Al-Qiyama which begins: 'I do swear by the Day of Resurrection...' (75) the Verses:-- 'Move not your tongue concerning (the Qur'an) to make haste therewith. It is for Us to collect it (Qur'an) in your mind, and give you the ability to recite it by heart. (75.16-17) Ibn ʿAbbas added: It is for Us to collect it (Qur'an) (in your mind), and give you the ability to recite it by heart means, "When We reveal it, listen. Then it is for Us to explain it," means, 'It is for us to explain it through your tongue.' So whenever Gabriel came to Messenger of Allah ﷺ ' he would keep quiet (and listen), and when the Angel left, the Prophet ﷺ would recite that revelation as Allah promised him.  

فِي قَوْلِهِ {لاَ تُحَرِّكْ بِهِ لِسَانَكَ لِتَعْجَلَ بِهِ} قَالَ كَانَ رَسُولُ اللَّهِ ﷺ إِذَا نَزَلَ جِبْرِيلُ بِالْوَحْىِ وَكَانَ مِمَّا يُحَرِّكُ بِهِ لِسَانَهُ وَشَفَتَيْهِ فَيَشْتَدُّ عَلَيْهِ وَكَانَ يُعْرَفُ مِنْهُ فَأَنْزَلَ اللَّهُ الآيَةَ الَّتِي فِي {لاَ أُقْسِمُ بِيَوْمِ الْقِيَامَةِ} {لاَ تُحَرِّكْ بِهِ لِسَانَكَ لِتَعْجَلَ بِهِ * إِنَّ عَلَيْنَا جَمْعَهُ وَقُرْآنَهُ} قَالَ عَلَيْنَا أَنْ نَجْمَعَهُ فِي صَدْرِكَ وَقُرْآنَهُ {فَإِذَا قَرَأْنَاهُ فَاتَّبِعْ قُرْآنَهُ} فَإِذَا أَنْزَلْنَاهُ فَاسْتَمِعْ {ثُمَّ إِنَّ عَلَيْنَا بَيَانَهُ} عَلَيْنَا أَنْ نُبَيِّنَهُ بِلِسَانِكَ قَالَ فَكَانَ إِذَا أَتَاهُ جِبْرِيلُ أَطْرَقَ فَإِذَا ذَهَبَ قَرَأَهُ كَمَا وَعَدَهُ اللَّهُ {أَوْلَى لَكَ فَأَوْلَى} تَوَعُّدٌ