The Qur'an was mentioned in the Previous Scriptures
Allah says;
وَإِنَّهُ لَفِي زُبُرِ الاْإَوَّلِينَ
And verily, it is in the Zubur of the former people.
Allah says;this Qur'an was mentioned and referred to in the previous Scriptures that were left behind by their Prophets who foretold it in ancient times and more recently.
Allah took a covenant from them that they would follow it, and the last of them stood and addressed his people with the good news of Ahmad;
وَإِذْ قَالَ عِيسَى ابْنُ مَرْيَمَ يبَنِى إِسْرَءِيلَ إِنِّى رَسُولُ اللَّهِ إِلَيْكُم مُّصَدِّقاً لِّمَا بَيْنَ يَدَىَّ مِنَ التَّوْرَاةِ وَمُبَشِّراً بِرَسُولٍ يَأْتِى مِن بَعْدِى اسْمُهُ أَحْمَدُ
And (remember) when `Isa, son of Maryam, said;"O Children of Israel! I am the Messenger of Allah unto you, confirming the Tawrah before me, and giving glad tidings of a Messenger to come after me, whose name shall be Ahmad. (61;6)
Zubur here refers to Books;
Zubur is the plural of Az-Zabur, which is also the name used to refer to the Book given to Dawud.
Allah says;
وَكُلُّ شَىْءٍ فَعَلُوهُ فِى الزُّبُرِ
And everything they have done is noted in the Az-Zubur. (54;52),
meaning, it is recorded against them in the books of the angels.
Then Allah says;
أَوَلَمْ يَكُن لَّهُمْ ايَةً أَن يَعْلَمَهُ عُلَمَاء بَنِي إِسْرَايِيلَ