The Four Books of Hadeeth [Al Kutub al Arba’ah]: [Also known as ‘The Sunan’]:

1. Sunan Abu Dawud
2. Sunan at Tirmidhee
3. Sunan an Nasaa-ee
4. Sunan ibn Majah.

[Al Hafidh ibn Katheer in Al Baahith al Hatheeth: 1/6]

The Five Books of Hadeeth [Al Kutub al Khamsah]:

They are the foundations of Islam, meaning :

1. Saheeh al Bukhari
2. Saheeh Muslim
3. Sunan Abu Dawud
4. Sunan at Tirmidhee
5. Sunan an Nasaa-ee

[Al Imam an Nawawee in Sharh Saheeh Muslim: vol 1: P.6.]

The Six Books of Hadeeth [Al Kutub as Sittah]: What became well known amongst the people of knowledge, is that the fundamental books that consist of the Ahadeeth of Ahkaam and others are:

1. Saheeh al Bukhari
2. Saheeh Muslim
3. Sunan Abu Dawud
4. Sunan an Nasaa-ee
5. Jaami’ at Tirmidhee
6. Sunan ibn Maajah

The people of knowledge are in agreement that the first five are from the fundamental books, and a lot of them considered Sunan ibn Maajah to be the sixth because it consists of a lot of Ahadeeth that were not collected in the five. [TN: Some of the people of knowledge considered Muwataa Maalik to be the sixth. The first scholar to add Sunan ibn Maajah to the list was al Hafidh abu Tahir al Andalosee]

It seems that the first scholar who used this terminology is al Hafidh al Haazimee.

The Order of the Six Books:

The first book is:

1. Saheeh al Bukhari, then
2. Saheeh Muslim, then
3. Sunan Abu Dawud, then
4. Sunan an Nasaa-ee, then
5. Sunan at Tirmidhi, and finally
6. Sunan ibn Maajah.

Some of the people of knowledge ordered them differently, giving some books precedence over others. It is mentioned in the biography of Abu Dawud as Sijistaanee – may Allah have mercy upon him – that Abu ‘Abdullah ibn Mandah said: “Those who separated authentic Ahadeeth from inauthentic Ahadeeth are four:

Al Bukhari and Muslim, after them comes

Abu Dawud and an Nasaa-ee.

The Nine Books of Hadeeth [Al Kutub at Tis’ah]:

As for the Nine Books of Hadeeth, it seems that the first people to use this terminology were the Mustashriqoon in their book ‘Al Mu’jam al Mufahras li Alfaadh al Hadeeth] They included the Six Books of Hadeeth and added:

This is why some of the scholars dislike using this term; ‘The Nine Books of Hadeeth’, because it was not used by the scholars of Hadeeth. So they hold the opinion that the scholars of Hadeeth should not use the terminology of the Mustashriqoon, since Hadeeth is not a science of theirs, rather this is not even from their religion.

Others held the opinion that there is no need to dislike using this term for it is only terminology…”

[Al ‘Allaamah ‘Abdul Muhsin al ‘Abbaad in Sharh Sunan Abu Dawud: 1/19]

[TN The most authentic book after the Quran is Saheeh al Bukhari, the two most authentic books are Saheeh al Bukhari and Saheeh Muslim.]