Muhammad had religious discussions with Jews and Christians
The allegation that these Jews and Christians were its source is groundless, especially since the role of Prophet Muhammad was merely that of a teacher; he openly invited them to embrace Islam, pointing out how they had deviated from God's true teaching of monotheism. Numerous Jews and Christians embraced Islam themselves upon hearing Muhammad’s message.

In addition, it was known that Muhammad was unlettered. In His divine wisdom, Allah chose His final Messenger to be an unlettered man so no one would have the slightest justification to doubt him or accuse him of writing or copying the Quran. Moreover, there was no Arabic version of the Bible in existence at the time of Prophet Muhammad. The earliest Arabic version of the Old Testament is that of R. Saadias Gaon of 900 CE - more than 250 years after the death of Muhammad .

We have mentioned that the Quran contains scientific miracles, and we presented a few facts stated within the book to evidence that it could not possibly have been known at that time by Muhammad or by any other person for that matter. More and more recent scientific discoveries are being found to coincide with what is stated in the Quran, and this is irrefutable evidence that its source was none but the all-knowing God - Allah, the Almighty.

https://www.youtube.com/watch?v=trinE1Vx_oY
https://www.youtube.com/watch?v=5Dx7yQUHOPo