I have heard Christians cite the following words from Sūrah al-Māʾidah, verse number 68, to argue that the Qurʾān tells them to follow their own book:

قُلْ يَـٰٓأَهْلَ ٱلْكِتَـٰبِ لَسْتُمْ عَلَىٰ شَىْءٍ حَتَّىٰ تُقِيمُوا۟ ٱلتَّوْرَىٰةَ وَٱلْإِنجِيل

{Say [O Prophet], “O People of the Book! You have nothing to stand on unless you observe the Torah, the Gospel…”}

This is only a part of the verse.

The full verse is as follows:

قُلْ يَـٰٓأَهْلَ ٱلْكِتَـٰبِ لَسْتُمْ عَلَىٰ شَىْءٍ حَتَّىٰ تُقِيمُوا۟ ٱلتَّوْرَىٰةَ وَٱلْإِنجِيلَ وَمَآ أُنزِلَ إِلَيْكُم مِّن رَّبِّكُمْ ۗ وَلَيَزِيدَنَّ كَثِيرًۭا مِّنْهُم مَّآ أُنزِلَ إِلَيْكَ مِن رَّبِّكَ طُغْيَـٰنًۭا وَكُفْرًۭا ۖ فَلَا تَأْسَ عَلَى ٱلْقَوْمِ ٱلْكَـٰفِرِينَ ۝٦٨

{Say [O Prophet], “O People of the Book! You have nothing to stand on unless you observe the Torah, the Gospel, and what has been revealed to you from your Lord [i.e., the Qurʾān].” And that which has been revealed to you [O Prophet] from your Lord will surely increase many of them in transgression and disbelief. So do not grieve over the disbelieving people.}

They leave out the part which mentions the Qurʾān because it does not support their claim.

In explanation of this verse, al-Ṭabarī (d. 310) states, “This is a command from Allāh, exalted be His mention, to His Prophet Muḥammad ﷺ to convey to the Jews and Christians who were in the midst of his place of emigration, [that] He, exalted be His mention, says to him {Say!} O Muḥammad to these Jews and Christians, {O People of the Book}; the Torah and the Gospel, {You have nothing to stand on} from that which you claim to stand on from what Mūsá came to you with O community of Jews, nor what ʿĪsá came to you with O community of Christians, {unless you observe the Torah, the Gospel, and what has been revealed to you from your Lord} from that which Muḥammad ﷺ came to you with from The Criterion (al-Furqān), acting on all of that, having faith in what it contains in terms of faith in Muḥammad ﷺ and believing in his truthfulness, and affirming that all of that is from Allāh, so do not disbelieve in any of it, and do not distinguish between the Messengers of Allāh thereby believing a part and disbelieving in a part, for indeed disbelieving in a part of that is disbelief in all of it, because the Books of Allāh confirm the truthfulness of each other, so whoever disbelieves in one of them has disbelieved in all of them.” [8: 572, ed. al-Turkī].

Lesson: whenever your opponent quotes something, do not take his word for it – look at the original source and read it yourself to ensure he has cited it accurately.

I ask Allāh to aid us over those who seek to cast doubt into the hearts of the believers.