There were some qualities that were essential to be found in the narrators of the hadith. If these qualities are not found in the narrator, the hadith given by those narrators will be rejected. The narrator of the hadith will be of firm faith. He should be honest. The truthfulness is also required in every aspect of his life. Besides that, he should have an excellent memory as well. He should be righteous and trustworthy. He should have sound knowledge of Quran hadith and fiqah and should know how meaning can be changed by changing a single word only.  It is also essential that he should report the exact words without using any word from his own. He should report what agrees with the reporters of others known to have good memories. 

It is essential for the narrators to follow what the Ummah has accepted and avoid to criticize it. There are examples of Imam Bukhari and Imam Muslim. If any of the narrators criticize Imam Bukhari or imam Muslim, his narrations will be considered fake narrations. There are two main parts of the hadith which are known as Isnad and Matan. There is a chain of narrators which leads towards companions of our holy Prophet Muhammad (Peace be upon him) and then finally to our holy Prophet Muhammad (Peace be upon him).  If Hadith is known as Hadith-e-Qudsi then it should be cleared that the text of the hadith is directly given by ALLAH Almighty. according to a Muslim scholar Abdullah bin Al Mubarak

Basically Isnad is a strict and rigorous analysis and in the absence of this rigor, anyone could say whatever he wanted. This strict analysis analyzes the links between individuals and Muhaddiseen until the link reaches the holy Prophet Muhammad (Peace be upon him). This practice was carried to authenticate the hadith.  

The Matan is basically the text of the hadith. The Matan will be considered as authentic if it is according to the Sunnah. The Matan is the text which is said by the Holy Prophet Muhammad (Peace be upon him). It should always be kept in mind that Matan should not be in the contradiction of the Holy Quran or Hadith. There are many times instructed in the holy Quran to offer prayers and to fast in the holy month of Ramazan but the detail that when to offer to pray and when to stop eating and when to start eating in Ramazan will be given in the hadith. 

There are some classifications of hadith as well. The Sahih hadith is free of all the doubts. The Hassan hadith is also fair and having very little doubt while Zaeef hadith is considered as weak hadith which may contain some doubts while Maudu is fabricated or rejected hadith.