Qur’anic Evidence

Alteration and Distortion (تحريف)
Allah says:
أَفَتَطْمَعُونَ أَنْ يُؤْمِنُوا لَكُمْ وَقَدْ كَانَ فَرِيقٌ مِنْهُمْ يَسْمَعُونَ كَلَامَ اللَّهِ ثُمَّ يُحَرِّفُونَهُ مِنْ بَعْدِ مَا عَقَلُوهُ وَهُمْ يَعْلَمُونَ
Translation:
"Do you then hope that they would believe you, while a party of them used to hear the Words of Allah and then distort it after they had understood it knowingly?"
Reference: Qur'an: 2:75

Writing Scripture With Their Own Hands
Allah says:
فَوَيْلٌ لِلَّذِينَ يَكْتُبُونَ الْكِتَابَ بِأَيْدِيهِمْ ثُمَّ يَقُولُونَ هَذَا مِنْ عِنْدِ اللَّهِ
Translation:
"So woe to those who write the Book with their own hands and then say, 'This is from Allah.'"
Reference: Qur'an: 2:79

Concealing and Changing Meanings
Allah says:
يُحَرِّفُونَ الْكَلِمَ عَنْ مَوَاضِعِهِ
Translation:
"They distort words from their proper places."
Reference: Qur'an: 4:46

Forgetting and Losing Revelation
Allah says:
فَنَسُوا حَظًّا مِمَّا ذُكِّرُوا بِهِ
Translation:
"They forgot a portion of what they were reminded of."
Reference: Qur'an: 5:13

The Qur’an as the Criterion (Muhaymin)
Allah says:
وَأَنْزَلْنَا إِلَيْكَ الْكِتَابَ بِالْحَقِّ مُصَدِّقًا لِمَا بَيْنَ يَدَيْهِ مِنَ الْكِتَابِ وَمُهَيْمِنًا عَلَيْهِ
Translation:
"And We have revealed to you the Book in truth, confirming what was before it and as a guardian over it."
Reference: Qur'an: 5:48
This proves the Qur’an corrects and exposes alterations in previous scriptures.
Evidence From Authentic Hadith

Prohibition From Relying on the People of the Book
The Prophet ﷺ said:
لَا تُصَدِّقُوا أَهْلَ الْكِتَابِ وَلَا تُكَذِّبُوهُمْ
Translation:
"Do not affirm the People of the Book nor deny them."
Reference: Sahih al-Bukhari: 4485
This shows their scriptures are unreliable.

Umar رضي الله عنه and the Tawrah
The Prophet ﷺ said when Umar read from the Tawrah:
أَمُتَهَوِّكُونَ فِيهَا يَا ابْنَ الْخَطَّابِ؟ وَالَّذِي نَفْسِي بِيَدِهِ لَقَدْ جِئْتُكُمْ بِهَا بَيْضَاءَ نَقِيَّةً
Translation:
"Are you confused about it, O son of al-Khattab? By Him in Whose Hand is my soul, I have brought it to you pure and clear."
Reference: Musnad Ahmad: 14736
Classed authentic by al-Albani.
Translation:
"Are you confused about it, O son of al-Khattab? By Him in Whose Hand is my soul, I have brought it to you pure and clear."
Reference: Musnad Ahmad: 14736
Classed authentic by al-Albani.
Statements of the Salaf

Ibn Abbas رضي الله عنهما
He said:
كَيْفَ تَسْأَلُونَ أَهْلَ الْكِتَابِ وَكِتَابُكُمْ الَّذِي أُنْزِلَ عَلَى رَسُولِ اللَّهِ أَحْدَثُ؟
Translation:
"How can you ask the People of the Book while your Book, revealed to the Messenger of Allah, is more recent?"
Reference: Sahih al-Bukhari: 7363

Imam Malik رحمه الله
He said:
"They have changed and altered, so it is not permissible to rely upon what is with them."
Reported by Ibn Abd al-Barr.
Reference: Jami' Bayan al-'Ilm: 2/42

Ibn Taymiyyah رحمه الله
He said:
"The Torah and Gospel that are in their hands today contain truth mixed with falsehood, alteration, and replacement."
Reference: Majmu' al-Fatawa: 4/104

Ibn al-Qayyim رحمه الله
He said:
"Allah exposed their lies, contradictions, alterations, and fabrications in their scriptures."
Reference: Hidayat al-Hayara: 1/31