90. Such sentences as this are usually found in the Quran at the end of a discourse so that it may end with the praise of Allah. It is plain from the context that the discourse ends with “the Sovereign.”
91. At the end of the discourse, the angel cautions the Prophet (peace be upon him) by Allah’s command to guard against a thing that had been noticed during the revelation, for it was not considered appropriate to do so during its recital. It is obvious from the wording of the caution that the Prophet (peace be upon him) was trying to learn the revealed discourse and was repeating it during its recital by the angel. Naturally this might have diverted his attention over and over again from listening to and grasping the revelation. Therefore it was felt necessary that he should be taught the right method of the receipt of the message of revelation and be asked not to try to commit it to memory during the revelation.
This sentence shows that this portion of Surah TaaHa is one of the earliest revelations, for we learn from the other early Surahs that the Prophet (peace be upon him) tried to commit to memory the Quranic revelation and Allah cautioned him against it. For instance in (Surah Al-Qiamah, Ayats 16-19), he was told: Do not move your tongue over and over again in order to learn the revelation, for it is Our responsibility to cause you to remember it by heart and recite it. Therefore when it is being recited to you, listen to it carefully. Then it is also Our responsibility to make its meanings clear to you.
Again in (Surah Al-Aala, Ayat 6), he was assured: We will teach you how to recite it and you will not forget it. It appears that afterwards when the Prophet (peace be upon him) had learnt how to receive the messages of revelation, this did not occur. That is why there is no such caution in the Surahs of the later period.