AS-SIHAHU ‘S-SITTAH.الستة الصحاح,
Also called al-Kutubu ‘s-Sittah الكتب الستة. “The six correct (books).” The title given to the six most trustworthy collections of traditions received by Sunni Muslims, namely, those by:—
(1) Abu ‘Abdi ‘llah Muhammad ibn Isma’il al-Bukhari, born A.H. 194; died A.H. 256.
(2) Abu ‘l-Husain Muslim ibn al-Hajjaj al-Qushairi, born A.H. 204, died A.H. 261.
(3) Abu ‘Isa Muhammad ibn ‘Isa ‘l- Tirmizi, born A.H. 209, died in 279 A.H..
(4) Abu Da’ud Sulaiman ibn Ash’a as-Sajastani, born A.H. 202, died in 215.
(5) Abu ‘Abdi ‘r-Rahman Ahmad ibn Shu’aib an-Nasa’i, born A.H. 215, died A.H. 303.
(6) Abu ‘Abdi ‘Ilah Muhammad ibn Yazid, ibn Yazid ibn Majah, al-Qazwini, born A.H. 209, died A.H. 273.
The above are generally esteemed the six authentic collections, but some substitute for the Sunan Ibn Majah the Muwatta’ of Abu ‘Abdi ‘llah Malik ibn Anas ibn Malik ibn Abi ‘ Amir ibn ‘Amr ibn al-Haris al-Asbahi al-Himyari, born A.H. 95, died A.H. 179.
(The above words in italics denote the popular title of the collection.)
Al-Bukhari and Muslim are held in highest reputation, and are called as-Sahihan, or “the two authentics.”
The collection by Malik, the founder of the second orthodox sect of the Sunnis, is the most ancient collection of traditions, and is held in high reputation, but it is sometimes omitted from the list by the Hanafis, because he is the founder of a certain school of jurisprudence. [TRADITIONS.]

Based on Hughes, Dictionary of Islam