How are you sure the Quran is authentic and the hadith isn’t when both were written after the prophet’s death?

 How are you sure the Quran is authentic and the hadith isn’t when both were written after the prophet’s death?

1.       Only fallible human records claim the Qur’an was not written during the Prophet’s time.
We do not know these accounts are true. More on this later.

 

2.       Written or not, the Qur’an is not what we find in a book.
The question reflects some common confusion about what the Qur'an is. The word "Qur'an" can be interpreted as 'the reading' or as 'the recitation'. It does not literally mean 'book' (that is "kitab").
The real PRESERVED Qur'an is not the Uthman copy, it's not the Tashkent copy, it's not any written or printed copy at all. Writings of the Qur'an are called a mus'haff.
The actual Qur'an lives in the memorized recitations that exist all over the world. There are thousands of websites at which you can listen to the recitation, or download it, or you can walk into almost every masjid and listen to a hafiz (Qur’an memorizer).
Linguists have compared the four major traditions of recitation (a process in which each student memorizer is taught and tested by ten separate instructors), and have found that they all match.
These four traditions, spread over four continents, ARE the Qur'an.

 

3.       The Qur’an proves itself to be the word of our Creator several ways.
Our Creator spells out the tests to give His book; tests no human book can pass. No one has found any failures. Not in fourteen centuries.
Try to imitate it, write ten chapters like it. Hud 11:13 
Human-written works can be imitated, God’s cannot.
Too difficult? Then try to write just one. al-Baqarah 2:23, Yunus 10:38
Again: human-written works can be imitated, God’s cannot.
Can’t do that either? OK, simply try to find any discrepancy in it. an-Nisa` 4:82
Any human-written work, especially one revealed over many years, will have many internal contradictions and/or incorrect facts;
there are none of these in the Qur’an.
The final test is the Qur’an itself - it’s effect on the sincere seeker when they hear it’s verses read, or read them personally. al-Ma`idah 5:83

 

4.       Now compare these qualities of the Qur’an to man-made hadith.
They can be - and have been - imitated.
That’s why hadith collectors struggled to classify them as ‘sahih’ (authentic), hassan (sound), da’if (weak) or maudu’ (forged).
They have many discrepancies, when compared against other hadith and when compared against the Qur’an.
Doubt this?
Find stoning of the married adulterer in the Qur’an.
Based on the word of our Creator such an act would be MURDER.

 

5.       The Qur’an calls itself ‘the best hadith’ (az-Zumar 39:23).
And tells us in the same surah (az-Zumar 39:18) that believers listen to what is said, and then follow the best.

 

6.       The Qur’an warns against later hadith.
See al-A`raf 7:185, al-Jathiyah 45:6, al-Mursalat 77:50, and the warning about alternative books in al-Qalam
“ 68:36 What is the matter with you? How do you judge? “
“ 68:37 Or do you have a Book which you study? “
“ 68:38 And in it do you find all that you wish for? “

Items 4, 5, and 6 tell us what we need to know about man-made material that tries to amend our understanding of our Creator’s book.

Take all such with a large grain of salt.