Scholars of Hadith state that the sahih (authentic) hadith which is most likely attributable to the Prophet (peace and blessings be upon him) is the hadith which fulfils all of the following five conditions:
1. Each of its narrators is of good character,
2. Each of its narrators has a precise memory,
3. The isnad (chain of narrators) is uninterrupted from the beginning to end,
4. The hadith is sound and free of any shudhudh (irregularity) in its isnad or matn (text), and
5. The hadith is sound and free of any ‘illah (fault) in its isnad or text.
The fourth and fifth conditions are among the most precise of conditions and the most difficult for the scholar, because proving them requires intense research and precision, bringing together all the isnads and narrations of the hadith, as well as extensive experience in the sciences of hadith and specialization in criticism.
Al-Hafiz ibn As-Salah (may Allah be pleased with him) says,
When scholars say “this hadith has a sahih isnad or a hasan isnad” instead of “this is a sahih hadith or a hasan hadith”, that is because it may be said that this hadith has a sahih isnad but it is not sahih per se because it is shadh (odd) or mu`allal (faulty).
Moreover, Ibn Kathir says,
The fact that the isnad is deemed to be sahih or hasan does not necessarily mean that the same applies to the text, because it may be shadh (odd) or mu`allal (faulty).
In his Alfiyah, Al-‘Iraqi says,
The ruling that the isnad is sahih or hasan does not necessarily apply to the text.
Nevertheless, there may be an exception to this differentiation if it is known that a particular imam does not make this distinction between the two terms “a sahih isnad” and “a sahih hadith” in his terminology. An imamsahih isnad” when he means that the hadith itself is sahih, and that it meets all five conditions. – especially if he is one of the earlier scholars – may say “a
Ibn As-Salah says,
A reliable scholar may say in his book “it (a hadith) has a sahih isnad” and does not mention any ‘illah (fault), or criticize it, so it may be understood that he deems it to be sahih in and of itself, because the absence of any ‘illah (fault) or qadih (flaw) is the basic principle.
Al-Hafiz Ibn Hajar (may Allah be pleased with him) says:
It seems to me that the correct view is that a distinction should be made between the one who differentiates when describing a hadith as sahih, stating it in either specific or general terms, and the one who does not do so. The one who is known from studying his books to make this distinction should be viewed accordingly, so when he speaks in general terms it should be understood as referring to both the isnad and the matn, and when he speaks in specific terms it should be understood as referring to the isnad only. And it maybe said concerning the one who is known to describe a hadith only in specific terms all the time what explored above.