ثُمَّ جَعَلْنَاكُمْ خَلَائِفَ فِي الْأَرْضِ مِنْ بَعْدِهِمْ لِنَنْظُرَ كَيْفَ تَعْمَلُونَ ﴿۱۴﴾ وَإِذَا تُتْلَى عَلَيْهِمْ آيَاتُنَا بَيِّنَاتٍ قَالَ الَّذِينَ لَا يَرْجُونَ لِقَاءَنَا ائْتِ بِقُرْآنٍ غَيْرِ هَذَا أَوْ بَدِّلْهُ قُلْ مَا يَكُونُ لِي أَنْ أُبَدِّلَهُ مِنْ تِلْقَاءِ نَفْسِي إِنْ أَتَّبِعُ إِلَّا مَا يُوحَى إِلَيَّ إِنِّي أَخَافُ إِنْ عَصَيْتُ رَبِّي عَذَابَ يَوْمٍ عَظِيمٍ ﴿۱۵﴾ قُلْ لَوْ شَاءَ اللَّهُ مَا تَلَوْتُهُ عَلَيْكُمْ وَلَا أَدْرَاكُمْ بِهِ فَقَدْ لَبِثْتُ فِيكُمْ عُمُرًا مِنْ قَبْلِهِ أَفَلَا تَعْقِلُونَ ﴿۱۶﴾

﴾14﴿ Thumma ja'alnaakum khalaaa'ifa fil ardi min ba'dihim li nanzura kaifa ta'maloon
﴾15﴿ Wa izaa tutlaa 'alaihim aayaatunaa baiyinaatin qaalal lazeena laa yarjoona liqaaa'a na'ti bi Qur'aanin ghairi haazaaa aw baddilh; qul maa yakoonu leee 'an 'ubaddilahoo min tilqaaa'i nafsee in attabi'u illaa maa yoohaaa ilaiya inneee akhaafu in 'asaytu Rabbee 'azaaba Yawmin 'Azeem
﴾16﴿ Wa izaa tutlaa 'alaihim aayaatunaa baiyinaatin qaalal lazeena laa yarjoona liqaaa'a na'ti bi Qur'aanin ghairi haazaaa aw baddilh; qul maa yakoonu leee 'an 'ubaddilahoo min tilqaaa'i nafsee in attabi'u illaa maa yoohaaa ilaiya inneee akhaafu in 'asaytu Rabbee 'azaaba Yawmin 'Azeem

﴾14﴿ We settled you (to test) on the earth after them to see how you would act
﴾15﴿ And when Our verses are recited to them clearly, those who do not believe in meeting Us say, Bring a Quran other than this or change it. Say, It is not for me to change it of my own accord. I only follow what is revealed to me. Indeed, I fear, if I were to disobey my Lord (by altering the Quran), the punishment of a great Day
﴾16﴿ If Allah had so willed, I would not have recited the Qur'an to you, nor would He have made you aware of it. Indeed, I have spent a long time among you before this; do you not then reason

[14] This verse also falls under worldly fear: (Kayfa ta‘malūn)—"How do you act?"—which is one of the purposes of the creation of Tawhid, as mentioned in verse 56 of Surah adh-Dhariyat.
And the second purpose is the path of servitude in accordance with the Sunnah, as indicated in this verse.
[15] This verse is also a warning to the deniers of Tawhid. The point is that when the signs of Tawhid are presented to them, they say: “The Qur'an is a good book of advice, but in every place there is a refutation of shirk—remove that part, and we’ll accept the rest.”
(Bayyinātin) refers to clear proofs that evidently indicate Tawhid.
(Ghayra hādhā aw baddilhu)—the difference is that ghayr (other than this) means: bring another book in place of this Qur'an and completely discard this one. Tabdīl (change) means: modify it—that is, remove the matter of Tawhid.
(Qul)—this is the first response to the demand of the polytheists. The point is: I cannot modify it, so I most certainly cannot replace it, as both are causes of severe punishment.
(In attabi‘u illā mā yūḥā ilayya)—this is proof that following anything other than revelation is not permissible.
(In ‘aṣaytu)—what is meant by disobedience here is altering or changing the revelation.
[16] This is the second response to the demand of the polytheists. The essence is that I did not initiate the delivery of this Qur'an from myself—its explanation by me and its teaching to you is by the will of Allah, the Exalted.
(Faqad labithtu)—The first point is that until the age of forty, you have known my truthfulness and trustworthiness. You also know that I am unlettered—I have neither read nor written anything. So how could I have composed this book on my own?
The second point is that it is evident to you that during these forty years, I never disobeyed Allah the Exalted—so how could I commit sin after forty years? This is a proof of his protection from sin (‘ismah) even before prophethood.
According to authentic hadith, prophethood began for the Prophet (May Allah bless him and give him peace) at the age of forty. Then he spent thirteen years in Makkah and ten years in Madinah, and he passed away at the age of sixty-three.
As for the narrations that mention sixty years, they have omitted the three (years).