What ḥadīth are
Ḥadīth (plural aḥādīth) are reports of what Muhammad said, did, or tacitly approved. Each ḥadīth has two parts: the isnād (chain of transmitters: X heard from Y heard from Z heard from the Prophet) and the matn (the content of the report itself). Together with the Qurʼān, the canonical ḥadīth shape the sunna — the prophetic example that Muslims are called to follow (Q 33:21). Sharia, jurisprudence (fiqh), Muslim ritual practice, family law, ethics, and most of the texture of Islamic life rests on ḥadīth more than on the Qurʼān itself. The Qurʼān tells you to pray; the ḥadīth tell you how.
The major collections
Sunni Muslims recognize the Kutub al-Sittah — the Six Books — as the most authoritative. They are: (1) Ṣaḥīḥ al-Bukhārī (d. 870, the most authoritative); (2) Ṣaḥīḥ Muslim (d. 875, second most); (3) Sunan Abū Dāwūd; (4) Sunan al-Tirmidhī; (5) Sunan al-Nasāʼī; and (6) Sunan Ibn Mājah. Bukhārī and Muslim together are called the Ṣaḥīḥān (the two sound ones). Twelver Shīʿa Muslims hold instead the Kutub al-Arbaʿa — the Four Books — beginning with al-Kāfī of al-Kulaynī. When citing a ḥadīth in conversation, naming the collection and number (Bukhārī 1) is the equivalent of citing a Bible passage with chapter and verse — it shows you are reading carefully, not summarizing.
The grading scale and how to cite carefully
Classical Muslim scholarship developed a sophisticated grading scale for ḥadīth based on the reliability of the isnād: ṣaḥīḥ (sound), ḥasan (good), ḍaʿīf (weak), and mawḍūʿ (fabricated). Within ṣaḥīḥ are sub-grades. Bukhārī (d. 870) reports having sifted some 600,000 ḥadīth down to roughly 7,400 entries he judged sound — the selection ratio is itself an admission that the great mass of ḥadīth was not trustworthy. Two practical rules for the Christian engager: (1) do not cite weak (ḍaʿīf) ḥadīth as if they were canonical, even if you find them online — your Muslim friend will (rightly) dismiss the citation; (2) when a friend cites a ḥadīth that troubles you, ask the collection and number, then look it up. It is much better to come back next week with a careful answer than to react in the moment.
Worked example
The moment
A Muslim friend says, the Prophet said... and quotes a ḥadīth you do not recognize. What do you do?
What you might say
"That is interesting. I do not know that one. Could you tell me which collection it is from? I want to look it up so I can give you a real answer next time, not a guess."
Why this works
The answer takes the friend seriously, names your honest ignorance, and protects you from confidently quoting back something inaccurate. I will look it up is one of the most credible things you can say in a Christian-Muslim conversation.
Watch out for
- Citing weak (ḍaʿīf) or fabricated (mawḍūʿ) ḥadīth as if they were authoritative. Always check the grade.
- Confusing canonical with authentic. A ḥadīth being in Bukhārī or Muslim does not by itself prove it is historically true; it means classical Muslim scholarship judged the chain of transmission to be sound.
- Treating ḥadīth as if they were Qurʼānic text. They are not. The classical tradition is careful about the difference.