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Conditions of a Sahih Hadith
A hadith is considered sahih (authentic) if it fulfills five conditions, provided that it has an isnad (chain of narrators) that meets five conditions.
The First Condition: Continuity of the Isnad
The first condition is the continuity of the isnad, meaning there should be no break in the chain. Each narrator should have heard the hadith directly from the one they are narrating from, thus avoiding any interruption, commentary, or indirect narration. This also excludes the mudallis (one who conceals a break in the chain) and those who have a hidden interruption.
The Second Condition: Justice and Reliability of the Narrators
The second condition is that all the narrators must be trustworthy and reliable, meaning they should retain what they narrate from the moment they heard the hadith until they narrate it. There should be no lapse in their memory, whether they memorize it in their mind or write it down. This is the main condition in this matter.
The Third Condition: Justice
The third condition is justice, meaning each narrator in the chain should not have committed a major sin that is evident, and they should avoid many minor sins. They should not be known for wickedness or the like.
The Fourth Condition: Absence of Shudhud (Anomaly)
The fourth condition is the absence of shudhud (anomaly). Shudhud occurs when a trustworthy narrator contradicts someone more reliable than them. If a narrator meets the conditions of reliability and justice, they may still make a mistake, as humans are not infallible. How do we identify their mistake? By comparing their narration with those of other reliable narrators.
If there is a difference in narration or the manner of narration, we prefer the one with better memory, or the majority, or other indicators that suggest their narration is more accurate than that of their opponents. The one whose narration contradicts is considered shudhud. Thus, the fourth condition is that there should be no shudhud in the isnad.
The Fifth Condition: Absence of Illa (Defect)
The fifth condition is the absence of illa (defect). Illa is a hidden reason that may not be apparent to many people regarding the authenticity of the hadith, but it affects its authenticity. Illa has many aspects in identifying it, such as following the chains of narration, then determining whether the matn (text) was preserved or not, and whether there was an error in the isnad or the matn. Because the illa may be in the isnad or in the matn.
These five conditions, if fulfilled by the hadith, make it sahih. We then say it is a sahih hadith.
The Difference Between "Sahih Isnad" and "Sahih Hadith"
Usually, if we say "its isnad is sahih," it means we have only considered the first three conditions, i.e., the isnad is connected, and all the narrators are just, reliable, and trustworthy. If these three conditions are met, we judge the isnad to be sahih.
Usually, if the scholar is well-versed in the science of hadith, they will also investigate the remaining two issues, which are illa and shudhud. If no shudhud or illa is found, they say, "Sahih isnad." For them, the hadith is sahih, as stated by the hadith scholar Ibn Hajar and others: that a sahih isnad should be acted upon unless an illa or shudhud is found, especially if one has not reviewed all the isnads or many of the isnads.
Thus, they say, "The isnad before me is sahih," meaning its isnad is connected with no interruption, the narrators are reliable, and the narrators are just. They are satisfied with these three conditions. Then, they study whether there is shudhud or illa. This requires more expertise and effort to search for shudhud and illa.
However, the more accurate view is that if a hadith scholar says "sahih isnad," it means the hadith is sahih; otherwise, they would mention its illa if there is one. They may say, "Its men are trustworthy, but it is marful (defective)," or they may say, "Sahih isnad, but it is marful," or they may say, "Sahih marfu' isnad," meaning the isnad's men are trustworthy, but it has an illa, such as marfu' (a hadith narrated by a successor from the Prophet peace be upon him without specifying that he heard it directly from him).
This is the detailed explanation of the difference between "sahih isnad" and "sahih hadith." And Allah knows best.