The bottom line is that bukhari and Co never ever mention or quote any “written sources” for their compilation. Most Hadith, if not all, start from hearsay and end with hearsay. A usual word is either “smi’tu” (I heard) or “’an” or “haddathana” all of which just mean that something was heard and relayed. To say that a hadith can be physically “verified” by determining credibility of people, most of them have been dead for tens of decades, is not science—even though they love to call it. It is still hearsay.