A Hadith may appear sound yet contain a hidden weakness affecting its soundness. This hidden defect is called ʿillah khafiyyah (علتِ خفیہ); a report with it is called muʿallal (معلل).
The fourth condition is that the report is not muʿallal.
An initially sound appearance is not enough; there must be no hidden defect affecting soundness.
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Ṣaḥīḥ li-Dhātihi: The Remaining Three Conditions
A Hadith may appear sound yet contain a hidden weakness affecting its soundness. This hidden defect is called ʿillah khafiyyah (علتِ خفیہ); a report with it is called muʿallal (معلل).
The fourth condition is that the report is not muʿallal.
An initially sound appearance is not enough; there must be no hidden defect affecting soundness.