Who vouched for the preservation of the Hadith, and are all the Hadith found in Bukhari and Muslim authentic?

The Sunnah (definition) must be preserved to preserve the Qur’an, because the Qur’an cannot be properly understood except through the Sunnah, for it explains and clarifies it. As for what Bukhari and Muslim narrated with a continuous chain of transmission, it is authentic, and the Ummah’s acceptance of it is sufficient. Muslim scholars have unanimously agreed on the authenticity of what is contained therein.