The Kutub al-Sittah

These are the six universally accepted books of hadith[1]. They are the saheeh of Imam Bukhari, the saheeh of Imam Muslim, the sunan of Imam Nasaai, the sunan of Abu Dawud, the Jami of Imam Tirmidhi and the the sunan of Ibn Majah. 

In summary there are two main aspects which can help to signify the authenticity of a hadith.

• Dabt – competency in terms of their understadning and narrating with accuracy
• Mulazaamat – time spent with teacher

We can summarise ahadith into five types: 

1. Katheer dabt, katheer mulazamaat – narrators are very competent and have spent a long time with their teacher
2. Katheer dabt, qaleel mulazamaat – narrators are very competent but have spent little time with their teacher
3. Qaleel dabt, katheer mulazamaat – narrators are not so competent but have spent a long time with their teacher  
4. Qaleel dabt, qaleel mulazamaat – narrators are not so competent and have spent little time with their teacher
5. Duafaa and majaahil – those who are weak and unknown

The authors of these six books have conditions on whether to include ahadith into their collections. In terms of their accepted authenticity, it will be as follows:

1. Bukhari – takes from (1) and some of (2)  (numbers refer to paragraph above)
2. Muslim – takes both (1) and (2)
3. Nasaai – takes from (1), (2) and (3)
4. Abu Dawud – takes from (1), (2) and (3), and uses (4) as supporting evidence, but mentions if it is reliable or not
5. Tirmidhi – takes from (1), (2), (3), (4) and occasionally (5)
6. Ibn Majah – takes from (1), (2), (3), (4) and (5) 

---

[1] There is ikthilaaf as to the inclusion of Ibn Majah in the list