FataAAala Allahu almaliku alhaqqu wala taAAjal bilqur-ani min qabli an yuqda ilayka (surah TaHa: 114)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(20:114) So, high and exalted is Allah, the True King *90 . And see that you do not hasten to recite the Qur'an before its revelation is completed to you, and pray, "O my Lord, give me more of knowledge. " *91

Fata`aalal laahul Malikul Haqq; wa laa ta`jal bil Quraani min qabli ai yuqdaaa ilaika wahyuhoo wa qur Rabbi zidnee `ilmaa

*90) Such sentences as this are usually found in the Qur'an 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 True King."
*91) At the end of the discourse, the Angel cautions the Holy Prophet 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 Holy Prophet 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 Ta Ha is one of the earliest Revelations, for we learn from the other early Surahs that the Holy Prophet tried to commit to memory the Qur'anic Revelation and Allah cautioned him against it. For instance in LXXV: 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 LXXXVII: 6, he was assured: "We will teach you how to recite it and you will not forget it." It appears that afterwards when the Holy Prophet 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.