HADITH TERMINOLOGY
صحيح
ṣaḥīḥ
The strongest grade — the chain is unbroken, and every narrator in it was both trustworthy and a reliable memoriser.
Scholars set five conditions: the chain is connected without a break; every narrator is ʿadl (upright in character); every narrator is ḍābiṭ (accurate in retention); the report is not shādhdh (not in conflict with better-attested versions); and it carries no ʿillah (no subtle hidden defect).
Ibn al-Ṣalāḥ defines it as a report "whose chain is connected through the transmission of the upright and precise from the upright and precise, to its end, and which is neither anomalous nor defective". Ibn Ḥajar adds the condition of complete precision (tāmm al-ḍabṭ) — and it is exactly that addition which distinguishes it from hasan.
Al-Nawawī gives a caution most readers miss: "when it is said ṣaḥīḥ, this is what is meant — not that it is certain; and when it is said not ṣaḥīḥ, the meaning is that its chain was not established as sound." So sahih is a statement about the strength of transmission evidence, not an absolute guarantee. It was also disputed whether later scholars may independently declare a hadith authentic: Ibn al-Ṣalāḥ said no; al-Nawawī explicitly disagreed, permitting it for one whose knowledge is well-grounded.