Accusations, insults, and mocking abound about Allah’s Messenger (ﷺ). Referred to as a poet, a magician, a soothsayer and even in our times someone who plagiarized the previous scriptures.
Plagiarism is the practice of taking someone else’s work or ideas and passing them off as one’s own.. This means that if one does not give credit for the source of the information then they are guilty of plagiarism.
Claims of Authorship
Historians, Orientalist, muslims and non-muslims all agree the man responsible for introducing, reciting, teaching and explaining the Quran to mankind was the Prophet Muhammad (ﷺ). What is differed upon is who was the original author. Regardless of ones view of who wrote the Quran the accusation of plagiarism relies on the claim by Prophet Muhammad (ﷺ) that he was the author. From a purely historical stand point no individual or group has made the claim that the Quran was a piece of literature written by their hand, which leaves only one source of evidence that we can turn to for evidence of authorship. The Quran itself.
Who is the Author of the Quran?
The One who claimed authorship repeatedly in the Quran is the same Who claimed authorship of the previous scriptures meaning as-Suhuf ‘Ibrahim, the revelation known as the Scroll of Ibrahim (Abraham ﷺ), at-Taurat, the revelation given to Musa (Moses ﷺ), az-Zabur the revelation given to Dauood (David ﷺ), and al-Injeel, the revelation known as the Gospel according to Isa (Jesus ﷺ).
And who believe in what has been revealed to you, [O Muhammad], and what was revealed before you, and of the Hereafter they are certain [in faith]. – Quran [2:4]
﴾ وَالَّذِينَ يُؤْمِنُونَ بِمَا أُنزِلَ إِلَيْكَ وَمَا أُنزِلَ مِن قَبْلِكَ وَبِالْآخِرَةِ هُمْ يُوقِنُونَ ﴿
Ibn `Abbas said that, (And who have faith in what is revealed to you and in what was revealed before you.) means, “They believe in what Allah sent you with, and in what the previous Messengers were sent with, they do not distinguish between (believing) them, nor do they reject what they brought from their Lord.’ 
And if you are in doubt about what We have sent down upon Our Servant [Muhammad], then produce a surah the like thereof and call upon your witnesses other than Allah , if you should be truthful. – Quran [2:23]
﴾ وَإِن كُنتُمْ فِي رَيْبٍ مِّمَّا نَزَّلْنَا عَلَىٰ عَبْدِنَا فَأْتُوا بِسُورَةٍ مِّن مِّثْلِهِ وَادْعُوا شُهَدَاءَكُم مِّن دُونِ اللَّهِ إِن كُنتُمْ صَادِقِينَ ﴿
The Revelation is From the Lord
Allah declares He is the author of the message in the sixty-ninth surat Al-Ĥāqqah (The Reality) – سورة الحاقة.
[That] indeed, the Qur’an is the word of a noble Messenger. – Quran [69:40]
﴾ انه لقول رسول كريم ﴿
And it is not the word of a poet; little do you believe. – Quran [69:41]
﴾ وما هو بقول شاعر قليلا ما تؤمنون ﴿
Nor the word of a soothsayer; little do you remember. – Quran [69:42]
﴾ ولا بقول كاهن قليلا ما تذكرون ﴿
[It is] a revelation from the Lord of the worlds. – Quran [69:43]
﴾ تنزيل من رب العالمين ﴿