The Qur'an was mentioned in the Previous Scriptures

Allah says:

وَإِنَّهُ لَفِي زُبُرِ الْأَوَّلِينَ ﴿١٩٦﴾

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.