Allāh declares in Qur'an 6:114-115; 18:27 that none — hence no human can alter his words as revealed through successive Prophets.
Most surely Allāh makes provisions for preventing the alteration of his authoritative words insomuch as they should bear witness unto the truth to every one who is of the truth. The question is how does Allāh preserve the waḥy revelations in his Holy Books throughout history despite human opposition?
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First of all there was an arrangement to record the revelations then and there in sheets. These were kept by the companions in piece meal. During the time of Prophet pbuh the method was not to take copy of these sheets and compile the same, but to get the revelations by heart. Thus there were a lot of companions who were hafizs. During the battle with Musailima the fake prophet 700 Sahabas were killed. Out of this 70 persons were hafizs. This shows that 10% of the population were hafizs. Thus the content of Holy Quran indisputably remained in the hearts of Muslims. This was the situation in the initial period. During the Caliphate period the necessity of compilation was felt. It started at the time of Caliph I and completed during the rule of Caliph III. Allah has granted political stability to the Islamic Nation to take up these kind of serious matters. After Prophethood, Caliphate actively played important role to complete important matters. Thus the text originally retained, later copied to several places and it is now spread as authorised copies with the silsila from the Prophet. In addition, there tens of thousands of hafizs are still alive so that any deviation in the text will be easily detected.