The six books Sahih Bukhari, Sahih Muslim, Sunan Abu Dawood, Sunan Nasai, Jami‘ at-Tirmidhi, and Sunan Ibn Majah are called “Sihah Sittah” (The Authentic Six). They are also called “Usul Sittah” or “Kutub Sittah.”

The first two books (Sahih Bukhari and Sahih Muslim) are called “Sahihain” and are not only authentic according to their authors but are held at the highest level of authenticity by the entire Ummah. According to Shah Waliullah Muhaddith Dehlawi (may Allah have mercy on him), anyone who objects to them is opposing the consensus of the Ummah and is innovating.

The last four books (Sunan Abu Dawood, Sunan Nasai, Jami‘ at-Tirmidhi, and Sunan Ibn Majah) are called “Sunan Arba‘ah.” Though they contain some weak Hadiths, due to the abundance of authentic narrations, most scholars include them in the “Sihah Sittah.”