SAHIH MUSLIM IS CONSIDERED THE MOST AUTHENTIC SOURCES OF ISLAM AND IS HIGHLY AUTHENTIC AND RELIABLE.
The footnote in Sahih Muslim vol.1 on p.1005 says here is disagreement on this. It says Prophet Mohammed himself participated in 26, and there were 56 others where he did not personally participate.
Ibn al-Hashimi’s Nasibi Imam Ibn Tamiyah confidently made the following admission in his book Majmo'a al-Fatawa, Volume 18 page 7:
فَلَيْسَ تَحْتَ أَدِيمِ السَّمَاءِ كِتَابٌ أَصَحُّ مِنْ الْبُخَارِيِّ وَمُسْلِمٍ
“There is’nt any other book under the surface of the sky more authentic than Bukhari and Muslim”
However when Muslims say Bukhari and Muslim are the most reliable books to learn about Islam, they are only talking about the fully connected chains of hadith.
We read in Hujatullah al-Balegha, Volume 1 page 249 by Shah Waliullah Dehalwi:
“The scholar agreed that all the connected traditions contained within both Sahihs are absolutely authentic and the two books are successively attributed to their (respective) authors and verily whoever belittled their status (the two books) is an innovator and is not adhering to the path of the believers.”
Muslim translator of Al-Bukhari's hadith collection:
It has been UNANIMOUSLY AGREED that Imam Bukhari's work is the most authentic of all the other works in Hadith literature PUT TOGETHER. The authenticity of Al-Bukhari's work is such that the religious learned scholars of Islam said concerning him: "The most authentic book after the Book of Allah (i.e., Al-Qur'an) is Sahih Al-Bukhari." .
Before he recorded each Hadith he would make ablution and offer two Rak'at prayer and supplicate his Lord (Allah). Many religious scholars of Islam tried to find fault in the great remarkable collection- Sahih Al-Bukhari, BUT WITHOUT SUCCESS. It is for this reason, they UNANIMOUSLY AGREED that the most authentic book after the Book of Allah IS Sahih Al-Bukhari. (Translation of the Meanings of Summarized Sahih Al-Bukhari, Arabic-English, translated by Dr. Muhammad Taqi-ud-Din Al-Hilali, Islamic University, Al-Madina Al-Munawwara, Kingdom of Saudi Arabia; compilation: Al-Imam Zain-ud-Din Ahmad bin Abdul-Lateef Az-Zubaidi [Maktaba Dar-us-Salam Publishers & Distributors, Riyadh-Saudi Arabia, 1994], pp. 18-19; bold and capital emphasis ours)
