Yaḥyá ibn Aktham reported;

“At a time when al-Mamūn was a prince, a Jewish man entered one of his gatherings. He was well-dressed, handsome looking with a profound aroma, and when he spoke, he spoke with impeccable eloquence. When the gathering concluded, al-Mamūn called him and asked, “Are you Jewish?” He replied, “Yes.”  “Accept Islam so that I may bestow favours upon you and reward you”, al-Mamūn said. “This is my religion, and the religion of my forefathers!” the man refused, then he departed.

The following year, he returned as a Muslim (and joined one of al-Mamūn’s gatherings) and began discussing matters of Islamic jurisprudence (fiqh). His speech was perfect. When the gathering ended, al-Mamūn called him and asked, “Are you not our friend from last year?” He said, “Indeed!” al-Mamūn then asked, “What was the reason for your conversion to Islam?” The man responded saying, “I left your gathering seeking to test all religions.  As you can see, I have beautiful handwriting. I came to the Torah and I wrote three copies whilst adding and removing parts of it. I took them to the Synagogue and they were purchased from me. I came to the Bible, and I wrote three copies whilst adding and removing parts of it. I took them to the Church and sold them. Then, I came to the Qurʾān and I wrote three copies whilst adding and removing parts of it.  I brought it to the (Muslim) copyists and bookbinders and they read through it. Once they had realised that it contained edits and omissions, they threw it away and they did not purchase it from me. And so I came to know that this book is protected, and this was the reason I became Muslim.”

I made ajj the same year. I met Sufyān ibn ʿUyainah and mentioned to him the story and he said, “This is supported in the Book of Allah.” I asked, “In which part?” He replied, “In the statement of Allah about the Torah and the Bible,

 ۚ إِنَّا أَنزَلْنَا التَّوْرَاةَ فِيهَا هُدًى وَنُورٌ ۚ يَحْكُمُ بِهَا النَّبِيُّونَ الَّذِينَ أَسْلَمُوا لِلَّذِينَ هَادُوا وَالرَّبَّانِيُّونَ وَالْأَحْبَارُ بِمَا اسْتُحْفِظُوا مِن كِتَابِ اللَّهِ وَكَانُوا عَلَيْهِ شُهَدَاءَ ۞
۞ فَلَا  تَخْشَوُا النَّاسَ وَاخْشَوْنِ وَلَا تَشْتَرُوا بِآيَاتِي ثَمَنًا قَلِيلًا  ۚ  وَمَن لَّمْ يَحْكُم بِمَا أَنزَلَ اللَّهُ فَأُولَٰئِكَ هُمُ الْكَافِرُونَ

Verily, We did send down the Torah [to Mūsá (Moses)], therein was guidance and light, by which the Prophets, who submitted themselves to Allah’s Will, judged for the Jews. And the rabbis and the priests [too judged for the Jews by the Torah after those Prophets] for to them was entrusted the protection of Allah’s Book, and they were witnesses thereto. Therefore fear not men but fear Me (O Jews) and sell not My Verses for a miserable price. And whosoever does not judge by what Allah has revealed, such are the Kāfirūn (i.e. disbelievers – of a lesser degree as they do not act on Allah’s Laws)” [Sūrah al-Māidah: 5: 44]

Allah put them in charge of protecting it, but its protection was lost.

Allah ﷻ  also said,

۞ إِنَّا نَحْنُ نَزَّلْنَا الذِّكْرَ وَإِنَّا لَهُ لَحَافِظُونَ ۞

“Verily We: It is We Who have sent down the Dhikr (i.e. the Qurʾān) and surely, We will guard it (from corruption)”  [Sūrah al-Ḥijr: 15: 9]

Allah ﷻ has protected it for us and it was not lost.

I say [al-Bayḥaqī]:  In the Qurʾān, and then from the statements of the Salaf, there is evidence [that demonstrates] that the previous nations, if they changed something from their religion, they would first change [contents of] their books, then they would have belief in this change with their hearts, then their desires, statements, and speech would conform. As for this nation, Allah has protected His Book and the Sunnah of his Prophet and He has made them firm in their belief of the Muslim so that they do not change anything from it, even though some people may have fallen into this out of desires or heedlessness. So all praise and thanks is due to Allah for protecting our religion, and all praise is due to Him for guiding us to it. We ask Him for steadfastness until death, and forgiveness on the Day of Gathering.