The earliest reports about Muhammad: Why are certain Hadith collections considered essential by maintream Muslim scholars of Islam?

In the Qur'an, "Allah" says at least 25 times that Muhammad is to be obeyed.

That is one reason why most Muslim scholars of Islam accept the Bukhari hadiths and a few other hadith collections as authoritative for Muslims: canonical hadiths show what Muhammad did and said, permitted and forbade.
Hadith is nothing but a reflection of the personality of the Prophet, who is to be obeyed at every cost.
That statement can be found on many, many Islamic sites on the web, including this one.

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It is impossible to understand the Qur'an without reference to the Hadith...
The above statement used to be at islamonline.net.  The quote can now be found at many places on the web, including here.

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The overwhelming majority of Muslims consider hadith to be essential supplements to and clarifications of the Qur'an, Islam's holy book, as well as in clarifying issues pertaining to Islamic jurisprudence
-- http://en.wikipedia.org/wiki/Hadith#Usage

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Muslim scholars of Islam separate hadith collections into various classes, often four: 1) authentic 2) good, 3) weak, 4) fabricated. The mainstream of Islamic scholarship places great credence in category 1. And as I don't wish to set up mere strawmen to knock down, I cite here at Quoting Islam hadiths from category 1 only.  In fact, I am even more selective than that: when I cite hadiths, I cite primarily the hadith collections that mainstream Islamic scholarship considers the crème de la crème within category 1, namely Bukhari's collection, Imam Muslim's collection, and once in a while Sunan abu-Dawud. Within category 1 are several other hadith collections, but they are on slightly lower rungs than the top-ranked Bukhari, or the second-ranked Imam Muslim, or the third-ranked Dawud.

A tiny percentage of Muslims are Qur'an-only Muslims who reject even the most canonical hadith collections. The mainstream of Islamic scholars considers Qur'an-only Muslims to be apostates. However, mainstream Muslim spokespersons speaking to non-Muslims will sometimes deny any significance to canonical hadiths. The denial may have several sources: 1) embarrassment about the content of a hadith, 2) a deliberate intention to deceive non-Muslims and give a false impression that the spokesperson would never support the ugly (to non-Muslims) values displayed in a hadith, or 3) general ignorance about hadiths.

Why do most believing Muslims take seriously the canonical hadith collections' reports about what Muhammad said and did?  Because Muslims take seriously the Qur'an's many commands to obey Muhammad. They also take seriously Qur'an 33:21:
Certainly you have in the Messenger of Allah an excellent exemplar for him who hopes in Allah and the latter day and remembers Allah much.
And no wonder Muslims take the Qur'an seriously: they consider the Qur'an the Incarnation, or rather Inlibration, of God. Christians often say the Bible is inspired by God.  Muslims do not believe the Qur'an is the result of anything so indirect or human-mediated as "inspiration" from God.  The Qur'an is taken to be the verbatim transcript of Allah addressing Muhammad.  The Qur'an's words are thought to exactly duplicate those in a book that has existed eternally with Allah in paradise.