For a Hadith to be acceptable/authentic, it has to meet 5 conditions:
- It must have a continuous and unbroken chain of narrators [FROM THE COMPILER, BACK TO THE PROPHET ﷺ]
- ALL of these narrators be trustworthy in their character and religion.
- These narrators have precised memories and make no mistakes. So they must also be trustworthy in their memory.
- The Hadith not contradict anything more authentic than it. If it does it is called Shadz (شاذ).
- There has no hidden defects (called 'Ilal (علل)) in the Hadith or its chain. —And this is the most difficult aspect to verify; Only a Hadith master has the capability of doing this.
Any Haidth that meets all of these five conditions is called Sahih (صحيح). —And that is the highest level of Hadith.
If however, one or more of the narrator(s) does not have as strong of memory as others, yet overall he is still reliable and he does have good character and an acceptable memory, then the Hadith will be lowered to the status of Hasan (حسن)/Good. But not as high as Sahih. So a Hasan Hadith differs from a Sahih Hadith in only one of these five conditions (which is, the 3rd condition). It is not that he is weak in his memory, it is just that he might not be as famous as those in the Sahih category (in term of trustworthiness in memory).
If on the other hand, one of these five conditions is not met, and yet at the same time there is no known fabricator or liar in its chain, the Hadith shall be categorized as Weak or even Very-weak —Dhaif (ضعيف) or Dhaif-jiddan (ضعيف جدا). And there are MANY different types of weak Hadith; over 30 different types of weak Hadith.
If a weak Hadith is reported from 2 different chains, (and in general each weak Hadith supports the other one), thus the Hadith itself is raised to a category of Hasan. We call it Hasan-li-ghairihi (حسن لغيره) or Hasan-due-to-external-factors.
A weak Hadith is of doubtful authenticity. We are not sure if the Prophet ﷺ said it or not. —This is in contrast to a Sahih or even a Hasan Hadith where we are pretty certain (or in fact in some cases certain 100%) that the Prophet ﷺ did indeed said that Hadith.
Obviously, if a Hadith is fabricated and Mawdu' (موضوع), there is no doubt in our minds that Prophet ﷺ did not say this Hadith.
Getting back to the topic at hand, we accept Sahih and Hasan Hadith; And we base our theology and our legal law on them. This is the case regardless of whether the Hadith has been narrated through one (Ahad (أحد)) or a few chains of narrations (Mutawatir (متواتر)). Of course there is no doubt that a Mutawatir Hadith is stronger than Ahad Hadith. And Ahad Hadith is not to the level of the Mutawatir. The point being that: an Ahad Hadith is not disqualified merely because it is Ahad. And this is the opinion of ALL of the scholars of Ahlil Sunnah Wal Jama'ah that: an Ahad Hadith can indeed be used for theology and for Fiqh. And there is some slight disagreement amongst the scholars in the finer details of applying this rule, but overall rule is agreed upon, and that is that: an Ahad Hadith is accepted and is something that we should act upon/believe in as long as it is authentic and it has met the five conditions listed above.
Regarding Weak and Very-weak Hadith, then by the unanimous consensus of the scholars, this cannot be used to base our theology on. Some scholars were a bit more lax about taking Weak Hadith in the performance of some voluntary-acts-of-worship. But there is no difference of opinion regarding the inadmissibility of using Weak Hadith for 'Aqidah/Theology. Therefore, any Hadith that is weak regarding our theology cannot and should not be taken as a source of our beliefs.
With this prelude to the actual science, the question that most of you might be asking is:
"How exactly do I know -as a layman- if a Hadith is
Sahih or Dhaif (is authentic or not)???"
Sahih or Dhaif (is authentic or not)???"
Well, to be very realistic and frank : there simply has to be an authority that a layman takes as his or her reference point. In other word: for the layman, there HAS TO BE an element of BLIND FOLLOWING.
Of course, this rule does not apply to Sahih Al-Bukhari and Sahih Muslim. These two books, -Bukhari and Muslim- are taken as authentic, and each and every Hadith in them, the lay person need not ask anyone else about them, they are authentic.
But this CANNOT BE APPLIED to ANY OTHER work of Hadith including the 4 sunan works: The Sunan of Abu Dawud, and At-Tirmizi, and An-Nasaie, and Ibn Majah. No doubt, these 4 books are -for the most part- more authentic than the other books of Hadith besides that Two Sahih works, yet, there are -in these collections- some Ahadith which are NOT authentic, particularly the sunan of Ibn Majah, it has a significant corpus of weak and very weak Hadith, and even some fabricated narrations.
So the question arises then, -the average layman has to follow an authority because he doesn't know how to analyze the Hadith himself-:
Who should he follow?
This is the question that each and every person needs to decide for himself, -if you wishes to-, have a reference point for Hadith.
Just like every lay person needs to find a scholar (or a few scholars) whom he takes as his reference for Fiqh, similarly, he should try his best to trust some of the specialist in Hadith.
And of course, of the greatest scholars of the past, known as the Khaatimal Huffaadh (The Seal of the Hadith Masters) is Al-Haafidh Ibn Hajar Al-Asqalani (died 852 A.H.), a great 'aalim and a great muhaddith. And he is the author of the most important explanation of Sahih Al-Bukhari called "Fath al-Bari" (and not just of Sahih Bukhari; it is the most important and authoritative explanation written on any Hadith book et al).
And there are many other scholars that are famous -past and present-. Of the ones who died in mid part of the last century is Shaykh Ahmed Shaker. Also in our times recently: Shaykh Muhammad Nasiruddin Al-Albani has done much work in Hadith and -in my humble opinion & the opinion of the vast majority of scholars as well-, his work is of extremely great benefit.
And of course the point being: the more that one studies the sciences of Hadith, the more one can judge by himself who he is qualified to take Hadith verdicts from.
Source: Shaykh Yasir Qadhi's lecture - The Mahdi ~Between Fact and Fiction~