The Islamic Personality by Sheikh Taqīuddīn An-Nabahānī

The khabar al-ahad (isolated report) in its three forms: gharib, 'aziz or mashhur, whether marfu', mawqūf or maqtu' is divided by the scholars of hadīth, in terms of its acceptance or rejection, into three categories: Sahih, hasan, da'eef. The following is a clarification of each category:

1. Sahih ; It is the hadīth whose isnād continues through the transmission of a trustworthy ('adl) narrator whose retention is accurate (dabit) from an another upright transmitter who has an accurate retentive ability until the end of the chain and is not shadh (irregular) or mu'allal (defective) i.e, the isnād of the hadīth is linked by the transmission of an upright ('adl) and accurate (dabit) narrator from someone similar to him until it ends with the Messenger of Allah  or ends with a Sahābi or someone else. The statement that the

excludes it from the mursal, munqati' and mu'dal ahadīth, which are not from the category of Sahih. Because the mursal is what the Tabi’een has narrated about the Prophet  without mentioning the Sahābi. The munqati' is when a single narrator is missing in one or more places in the isnād. The mu'dal has two or more narrators missing from one or more places in the isnād. All of them, that is the mursal, munqati' and mu'dal have discontinued isnāds which takes it out of the Sahih category. The statement that;

excludes the Sahih hadīth from the shadh report where a trustworthy narrator goes against the transmissions of narrators who are more reliable than him. The statement:

excludes the Sahih ahadīth from the mu'allal report which has a defect. The 'illah (defect) consists of a denigratory thing in the hadīth effecting its rejection, which appears to the rijal critics when collecting and collating the various transmission routes of the hadīth, such as the chain of a narrator being continuous while a group has transmitted as mawqūf i.e attributed it to a Sahabi.

The statement:

It excludes the report narrated by a transmitter whose apparent and hidden condition is not known, majhul al-'ayn, or the transmitter is known to be weak, such a hadīth is not considered as Sahih.

The statement:

2. Hasan: It is a report that is known by the one who reported it and its transmitters became well known and consequently it is the core of most of the hadīth. It is a hadīth most scholars accept and it is used by the fuqaha generally i.e, that in the isnād there are no narrators that have been charged with lying and it is not a shadh (irregular) hadīth. These are two types:

First: a hadīth whose isnād is not free from transmitters who are mastur (of hidden conditioin) and whose capacity is not realised. However they are not negligent and are not prone to make mistakes and nor are they charged with mendacity. The matn of the hadīth may have been narrated by someone at a similar level to him due to which it will not be included as shadh or munkar (rejected). Second: The narrators must be known for their honesty and trustworthiness but they do not attain the level of the transmitters of the Sahih category in retention and exactitude. A narrator who is alone in transmitting a report is not considered as munkar (rejected) and nor is the matn irregular (shadh) or defective (mu'allal). So the hasan ahadīth is the report transmitted by an upright ('adl) narrator who is of lesser retentive capacity, whose isnād is continuous and not irregular (shadh) or defective (mu'allal), the hasan hadīth is used as proof exactly as the Sahih hadīth is used.

3. Da'eef: It is the hadīth which does not have the qualifications of the Sahih or hasan ahadīth. The Da'eef (weak) hadīth is not used as evidence at all. It is a mistake to say that when a da'eef hadīth comes via numerous lines of transmission then it rises to the level of hasan or Sahih. For when the hadīth is weak this means the narrators have actually committed transgressions or have been accused of lying. When the hadīth has come through other lines of transmission which are of this type, then it has increased in its weakness. As for when the meaning contained in the da'eef hadīth is also contained in the Sahih ahadīth, then the Sahih hadīth is cited and the da'eef hadīth is disregarded. Therefore, the da'eef hadīth is not used as proof in any way whatsoever.