Depending on soundness and judgment, hadiths, in fact ahad hadiths, are classified into three groups: sahih, hasan and da'if.

These are the hadiths that are not shadhdh (irregular) or mu'allal (defective) and are reported by just and trustworthy narrators with a muttasil sanad. If a sahih hadith has all those characteristics to the highest degree, it is called sahih li zatihi. Mutlaq (definitely) sahih  is the same as sahih li zatihi. 

Hadiths that do not have these characteristics of reliability to the highest degree, but are taken to a degree of reliability by another narration are called sahih li gayrihi.

Scholars of hadiths categorize hadiths into seven grades:

a. Those which are transmitted by both al-Bukhari and Muslim. They are also called muttafakun alayh. The number of hadiths of this kind in the book el-Lu'lu'u ve'l-mercân by M. Fuad Abdülbâkî is 1,906.

b. those which are transmitted by al-Bukhari only;

c. those which are transmitted by Muslim only;

d. those which are not found in either of these two collections, but agree with the requirements of both al-Bukhari and Muslim;

e. those which agree with the requirements of al-Bukhari only;

f. those which agree with the requirements of Muslim only;

and

g. those declared to be sahih by other hadith scholars.

According to soundness, the second category of hadiths is hasan hadiths. These are the hadiths that not shadhdh or mu'allal, and which are reported by not perfectly precise narrators with muttasil sanads. This kind of hadith is called hasan lizatih and ranks with the grade of sahih li gayrih if its words are reinforced by another hadith.

The hadith is called hasan li gayrih if it is reinforced in meaning or words by other hadiths whose narrators have not been accused of deception or who are not forgetful and make a lot of mistakes, but their competence is not obvious (mastur narrators).

Hadiths that do not meet the above-mentioned requirements of sahih or hasan hadiths are "da'if." The weakness of a hadith stems from one of two reasons:

1) inqita' (discontinuity) in the sanad,

2) or the narrator having a disparaged character.

Inqita' means that there is at least one missing link in the sanad. If there is such an inqita', this inqita' necessitates the rejection of the text, even if all other narrators are siqa'. Because of inqita, da'if hadiths are mursal (hurried), munqati‘ (broken), mu'dal (unclear), mu'allaq (hanging), or mudallas (concealed).

Mursal hadiths are those quoted by one of the following generation directly in the name of the Prophet without the name of any of the Companions being mentioned.

Munqati‘ hadiths are those whose sanad is not muttasil. Hadiths which have one or more than one narrators missing, but not successively, in the sanad are called munqati.

Mu‘dal hadiths are those in which two or more narrators are missing successively in the sanad. This kind of hadiths is weaker than munqati hadiths.

Mu‘allaq hadiths are those in which one or more than one narrators are not known at the beginning of the sanad or none of the narrators are known up to the muntaha of the hadith. Narrations that are included in recent hadith books intended particularly for the general public and cited with the sole mention of a Companion are all mu'allaq. However, they do not lose their reliability because their sanads are muttasil in the original sources.

Tadlis (concealing) refers to a sanad where a narrator is concealed as if there was no such a person. The hadith narrated with tadlis is called mudallas. There are three types of tadlis:

1. tadlis al-isnad. A narrator reports from a contemporary whom he did not meet or met but did not hear the hadith from, in such a way as to create the impression that he heard the hadith in person.
2. tadlis al-shuyukh. The narrator does mention the one he transmitted from by name, but uses a less well-known name, by-name, or nickname.
3. tadlis al-taswiyyah. A hadith is narrated as if all its narrators are siqa', and the intermediate weak authority is omitted.

Da'if hadiths due, to the disparaged character of the narrator, are categorized into 10 types: Maudu', matruk, munkar, mu'allal, mudraj, makloob, mudtarib, shazz, musahhaf and muharraf.

Maudu‘ is a hadith whose narrator has been accused of telling lies in relating hadith.

A matruk or matruh is a hadith whose narrator is accused of falsehood and being alone in this narration. It is also defined as a hadith transmitted by only one narrator who has been accused of deception, falsehood, fisq (departure from the path of Islam) and negligence, although it is not contrary to any narration of trustworthy narrators.

There are different views on the description of munkar hadith, but there are two main views:

a. A hadith that has a narrator, although weak that contradicts the narration of trustworthy narrators.

b. A hadith that has only one narrator, whether he is trustworthy or not.

Mu'allal or ma‘lul are hadiths that are ostensibly sahih but have a concealed defect that detracts from its soundness. The hadith scholar who discovers this defect is called mu'allil.

The mentioning of a remark that does not belong to a hadith is called idraj and a hadith that is subject to such a practice is called mudraj. This means that words of a narrator have become confused with the words of the Prophet. Idraj can be placed in the sanad or matn.

Maqlub is a hadith in which the names of the narrators in the sanad or some words in the matn are transposed.

Mudtarib is a hadith that, although having more than one narration, there is no reason for preferring one over the others. Izdirab generally takes place in isnad or sometimes in matn.

Shazz is the narration of a trustworthy narrator that contradicts those of more trustworthy narrators. The narration of the more trustworthy narrator is called mahfuz. It can be placed in the sanad or matn.

Musahhaf is a hadith in which the words are wrongly placed. This kind of mistake generally occurs while written documents are being copied.

Muharraf is a hadith in which the diacritical marks are wrongly placed.