Narrated Ibn `Abbas: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)
حَدَّثَنَا الْحُمَيْدِيُّ، حَدَّثَنَا سُفْيَانُ، حَدَّثَنَا مُوسَى بْنُ أَبِي عَائِشَةَ ـ وَكَانَ ثِقَةً ـ عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم إِذَا نَزَلَ عَلَيْهِ الْوَحْىُ حَرَّكَ بِهِ لِسَانَهُ ـ وَوَصَفَ سُفْيَانُ ـ يُرِيدُ أَنْ يَحْفَظَهُ فَأَنْزَلَ اللَّهُ {لاَ تُحَرِّكْ بِهِ لِسَانَكَ لِتَعْجَلَ بِهِ}
Bukhari
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) • Hadith 447