This question can come from non Muslim or even some Muslims who lack basic knowledge about Islam.
For non Muslims we say that Quran is living and ever lasting miracle in Islam which challenges every one to bring like any part of Quran. No one from all the enemies of Allah could bring any verse like Quran. This proves that Quran is not for a human source . Quran clearly states that Islam is the last religion and Prophet Mohammad (SAWA) and no Prophet will come after him.( Sura 33, Verse 40).
The Prophet Muhammad (SAWA) has made thousand of statements during 23 years of his declared prophethood. None of his statements was wrong at all. He himself said: O Ali, you are from me like Haroun was to Easa, but no Prophet will be after me.( Saheeh al-Bukhari , Hadeeth number 4064).