To sum up, one of the major reasons for the difference of opinion found among mujtahids is that different Imams have different criteria for establishing the reliability of a hadith. This is why the scholars of hadith and principles of hadith [usul al-din] consider it imperative to teach the principles of hadith before teaching the actual books of hadith. This enables the student to understand the reasons why some hadiths have been disregarded. At the same time, he will understand why scholars, in spite of an explicit proposition appearing in a hadith, act contrary to it.
This is why it has always been my fervent desire to request that the people who teach translations of the hadiths would at least teach a brief summary of the principles of hadith. This would ensure that ordinary people, who zealously read hadiths in translations because they are the speech of Allah’s Messenger (may Allah send peace and blessings upon him), are not lead astray. It would also ensure that they do not become disinclined to the juridical rulings [masa’il fiqhiyya], and that they do not develop any evil thoughts about the hadiths. All of these factors can lead to a fatal deficiency in their faith [din]. And Allah guides whom He wills to the straight path.
The author of Tadhkira writes, “There is a very intricate and difficult issue in the field of hadith that, since the fabricators and the preachers have fabricated many hadiths, and many other pious people have misunderstood the meaning of the hadiths, the mujtahids felt a need to establish a set of principles to assess the validity of a hadith. This set of principles will differ from principles set up by the Imams of Hadith.”
There are also other factors which render a hadith unreliable. Until a person has knowledge about them, it is not permissible for him to practice upon hadiths directly.
(The Differences of the Imams, Shaykh al-Hadith Zakariyya Khandhlawi)