It is well-known that no report, in the science of hadith, is accepted unless it gives the full chain of narrators upto the Holy Prophet (SAWS). Each narrator from this chain is first scrutinized on the touch-stone of his credibility as discussed above. But even if all the narrators of a chain are found to be reliable, it is not enough to hold the tradition as authentic. It must be proved that the chain is constant and no narrator has been missed in between. If it is found that some narrator has been missed at any stage, the tradition is held to be unreliable. To ensure the constancy of the chain, it is necessary to know about each narrator whether it is possible for him historically to meet the person from whom he claims to hear the tradition.
This scrutiny is indeed very difficult and delicate. But the scholars of the science of hadith have undertaken this task in such an accurate manner that one cannot but wonder.
While holding an enquiry about each narrator, the scholars, besides ascertaining his integrity and memory, would also survey his teachers and pupils. Thus, a detailed list of both his teachers and pupils is available in each detailed book of Rijal. So, when deciding about the constancy of a hadith the scholars do not only make themselves sure about the dates of birth and death of each narrator, but also examine the list of his teachers and pupils.
Not only this, they often try to fix the time-span in which a narrator had opportunities to meet a particular teacher and that in which he did actually hear Ahadith from him. On the basis of this information they derive certain important conclusions about the credibility of a narrator.
For example, ‘Abdullah ibn Lahi’ah is a well-known Egyptian narrator of hadith. It is established that his memory was weak and he used to narrate those traditions which he wrote. At a particular time, his house was burnt by fire and all his books were also burnt. After this occurrence he sometimes used to report Ahadith from his memory. Therefore, some scholars have decided that his narrations before the accident are reliable while those narrated after it are not worthy of trust. Now, the pupils who have heard Ahadith from him in the early period, their narrations may be accepted, while the reports of those who have heard from him in the later period cannot be relied upon. The scholars have scrutinized the list of his pupils and have specified the names of his early pupils like ‘Abdullah ibn Wahb etc. and have declared that all the rest should be treated as his later pupils, and no trust might be placed on their narrations.
In short, the second type of scrutiny, which is very essential in the criticism of traditions, relates to the constancy and perpetuity of the chain of narrators. If it is found that a narrator has not heard the hadith directly from the one to whom he is ascribing it, the tradition is said to be Munqati’ (broken) which cannot be treated as reliable.
The third test applied to a tradition relates to its comparison with what is narrated by other pupils of the same teacher.