Who was the first person to compile sahih hadith, and what are the levels of sahih hadith?

Summary

Based on the answer cited from Sheikh Mustafa al-‘Adawi, the first person to give attention to compiling sahih hadith was Imam al-Bukhari, through his Sahih al-Bukhari, written at the suggestion of his teacher Ishaq bin Rahawaih. A group of scholars ranked sahih hadith from the highest, namely what al-Bukhari and Muslim agreed upon, then narrations of al-Bukhari alone, of Muslim alone, hadith meeting the conditions of both, hadith meeting the sahih conditions set by each author in his book, and sahih hadith not recorded by either imam.

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