Al-Jawāb as-Sahīh li-man baddala dīn al-Masīh - Ibn Taimiya - (2/270)

The reply:
Once you recognise that the wording of the opening verse of Sūrah al-Māʾidah is,
“We have sent down to you the Book in truth, confirming whatever Scripture was before it and acting as a guardian over it” ⦗5:48⦘,
then it may be said:
As for Muhammad, the Seal of the Messengers—may Allah bless him and grant him peace—his affirmation of all the Scriptures Allah revealed before him and of the prophets who came before him is something necessarily known from his religion. It has been conveyed through recurrent, mass-transmitted reports (tawātur) as evident as the tawātur of his being sent to all creation, and this forms one of the fundamentals of faith.
Allah, Exalted is He, says:
“Say, ‘We believe in Allah, and in what has been sent down to us, and in what was sent to Abraham, Ishmael, Isaac, Jacob and the Tribes; and in what Moses and Jesus received; and in what the prophets received from their Lord. We make no distinction between any of them, and to Him we submit.
So if they believe in the same as you have believed, they are rightly guided; but if they turn away, they are only entrenched in discord. Allah will suffice you against them, and He is the All-Hearing, the All-Knowing.’” ⦗2:136-137⦘
He, the Most High, also says:
“Say, ‘We believe in Allah, and in what has been sent down to us, and in what was sent to Abraham, Ishmael, Isaac, Jacob and the Tribes; and in what Moses and Jesus received; and in what the prophets received from their Lord. We make no distinction between any of them, and to Him we submit.
Whoever seeks a religion other than Islam, it will never be accepted from him, and in the Hereafter he will be among the losers.’” ⦗3:84-85⦘

وَالْجَوَابُ: بَعْدَ أَنْ تَعْرِفَ أَنَّ لَفْظَ الْآيَةِ الْأُولَى مِنْ سُورَةِ الْمَائِدَةِ {وَأَنْزَلْنَا إِلَيْكَ الْكِتَابَ بِالْحَقِّ مُصَدِّقًا لِمَا بَيْنَ يَدَيْهِ مِنَ الْكِتَابِ وَمُهَيْمِنًا عَلَيْهِ} [المائدة: ٤٨] . أَنْ يُقَالَ: أَمَّا تَصْدِيقُ خَاتَمِ الرُّسُلِ مُحَمَّدٍ رَسُولِ اللَّهِ - صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ - لِمَا أَنْزَلَ اللَّهُ قَبْلَهُ مِنَ الْكُتُبِ وَلِمَنْ جَاءَ قَبْلَهُ مِنَ الْأَنْبِيَاءِ فَهَذَا مَعْلُومٌ بِالِاضْطِرَارِ مِنْ دِينِهِ مُتَوَاتِرًا تَوَاتُرًا ظَاهِرًا كَتَوَاتُرِ إِرْسَالِهِ إِلَى الْخَلْقِ كُلِّهِمْ وَهَذَا مِنْ أُصُولِ الْإِيمَانِ. قَالَ - تَعَالَى -: {قُولُوا آمَنَّا بِاللَّهِ وَمَا أُنْزِلَ إِلَيْنَا وَمَا أُنْزِلَ إِلَى إِبْرَاهِيمَ وَإِسْمَاعِيلَ وَإِسْحَاقَ وَيَعْقُوبَ وَالْأَسْبَاطِ وَمَا أُوتِيَ مُوسَى وَعِيسَى وَمَا أُوتِيَ النَّبِيُّونَ مِنْ رَبِّهِمْ لَا نُفَرِّقُ بَيْنَ أَحَدٍ مِنْهُمْ وَنَحْنُ لَهُ مُسْلِمُونَ - فَإِنْ آمَنُوا بِمِثْلِ مَا آمَنْتُمْ بِهِ فَقَدِ اهْتَدَوْا وَإِنْ تَوَلَّوْا فَإِنَّمَا هُمْ فِي شِقَاقٍ فَسَيَكْفِيكَهُمُ اللَّهُ وَهُوَ السَّمِيعُ الْعَلِيمُ} [البقرة: ١٣٦ - ١٣٧] وَقَالَ - تَعَالَى -: {قُلْ آمَنَّا بِاللَّهِ وَمَا أُنْزِلَ عَلَيْنَا وَمَا أُنْزِلَ عَلَى إِبْرَاهِيمَ وَإِسْمَاعِيلَ وَإِسْحَاقَ وَيَعْقُوبَ وَالْأَسْبَاطِ وَمَا أُوتِيَ مُوسَى وَعِيسَى وَالنَّبِيُّونَ مِنْ رَبِّهِمْ لَا نُفَرِّقُ بَيْنَ أَحَدٍ مِنْهُمْ وَنَحْنُ لَهُ مُسْلِمُونَ - وَمَنْ يَبْتَغِ غَيْرَ الْإِسْلَامِ دِينًا فَلَنْ يُقْبَلَ مِنْهُ وَهُوَ فِي الْآخِرَةِ مِنَ الْخَاسِرِينَ} [آل عمران: ٨٤ - ٨٥]