Kutub al-Sittah

The Kutub al-Sittah () are six (originally five) books containing collections of "hadith" (sayings or acts of the Islamic prophet Muhammad) compiled by six Sunni Muslim scholars in the ninth century CE, approximately two centuries after the death of Muhammad. They are sometimes referred to as al-Sahih al-Sittah, which translates as "The Authentic Six". They were first formally grouped and defined by Ibn al-Qaisarani in the 11th century, who add Sunan ibn Majah to the list. Since then, they have enjoyed near-universal acceptance as part of the official canon of Sunni Islam.
Not all Sunni Muslim jurisprudence scholars agree on the addition of Ibn Majah. In particular, the Malikis and Ibn al-Athir consider al-Muwatta' to be the sixth book. The reason for the addition of Ibn Majah's Sunan is that it contains many Hadiths which do not figure in the other five, whereas all the Hadiths in the Muwatta' figure in the other Sahih books.
Significance.
Sunni Muslims view the six major hadith collections as their most important, though the order of authenticity varies between Madhhabs:
The first two, commonly referred to as the "Two Sahihs" as an indication of their authenticity, contain approximately seven thousand hadiths altogether if repetitions are not counted, according to Ibn Hajar.
Authors.
According to the "Cambridge History of Iran": "After this period commences the age of the authors of the six canonical collections of Sunni hadith, all of whom were Persian, except Imam Malik. The authors of the six collections are as follows: