I've read a fair amount of Islamic scripture over the last few months, and this doesn't read like anything I've encountered before. I don't get the impression, from the scriptures he left behind, that Muhammad was a man inclined towards lengthy prose compositions--he seemed to prefer epigrammatic pronouncements and verse poetry. The reference to "black" and "white" as ethnic groups struck me as a little anachronistic, but it could equally just be an unorthodox translation. There are other things that made me suspicious of this text from the start, the biggest one was the fact that these texts are almost identical, they've got the same caps or boldface for emphasis, the same phrasing, and the exact same words. To me this smells of unthinking copy pasting, like an endlessly forwarded email.
The second thing that bothers me is the citation that most of these texts have at the bottom. It goes like this --
See Al-Bukhari, Hadith 1623, 1626, 6361
Sahih of Imam Muslim also refers to this sermon in Hadith number 98.
Imam al-Tirmidhi has mentioned this sermon in Hadith nos. 1628, 2046, 2085.
Imam Ahmed bin Hanbal has given us the longest and perhaps the most complete version of this sermon in his Masnud, Hadith no. 19774.
The references are fascinating. They look authoritative at first glance---they mention real Hadith collections and are formatted in the right way---but if you subject them to a moment's scruntiny they undermine the authenticity of this text more than they preserve it.
Hadith collections are not exactly scripture, they're probably best described as reasonbly trustworthy anecdotes about the prophet for use in a theological tiebreaker scenario. Where Chrstianity has Matthew, Mark, Luke, and John, Islam has the major hadith scholars. Both sets of texts were compiled a long time after the events they describe, and both are fourth- or fifth-hand information. The big difference between the two is that people like Muhammad al-Bukhari and Muslim ibn al-Hajjaj documented their sources and didn't make any claims that their texts were, ahem, gospel. As a result, hadith collections are huge and messy. For the sake of completeness, Hadith scholars often include several versions of the same story, sometimes dozens. Each version has a different chain of narrators (known as an isnad) and reached the collector from a different source. What this means is that three separate hadith about the same event, even it they're from the same collection, are likely to differ. Sometimes this difference is minor -- just a few words changed -- but quite often it's a completely contradictory account. If there was just one citation, perhaps two, then this might be authentic, but this many suggests that, at best, the text has been cobbled together and inferred from several fragments.
All of this, of course, assumes that the hadith contain a full or even partial text of the sermon. The last reference, interestingly, says that one of the hadith is "perhaps the most complete version," suggesting that the others are just fragments or passing references. Hadith collections include all the information that the hadith collector thought seemed authentic enough, regardless of whether it was interesting or useful. This means that some hadith document amazingly banal everyday conversations with no obvious theological importance other than that one of the speakers was Muhammad. They also include incredibly vague narrations. It is quite possible that a hadith that "mentions" the farewell sermon may consist of something like:
Narrated Big Dave
"I heard someone say that the prophet gave some sort of sermon when they all went to Mecca. I have no idea what he said though, sorry."
The last, and most important part of this is the Hadiths actually say. (It took me a while to track them down because not many online hadith collections sort them by number. As a result, I'd written the previous paragraphs without knowing the content of the hadiths. I would go back and rewrite the whole thing, but I'm lazy and that feels too much like work.)
Here are the sources mentioned, and what they actually say --
Sahih Bukhari 1623. Narrated Ibn 'Umar:
Allah's Apostle (SallAllaahu `Alayhi Wa Sallam) (got) his head shaved after performing his Hajj.
Sahih Bukhari 1626. Narrated 'Abdullah:
The Prophet and some of his companions got their heads shaved and some others got their hair cut short. Narrated Muawiya: I cut short the hair of Allah's Apostle with a long blade.
Sahih Bukhari 6361. Narrated Abdullah:
Allah Apostle said in Hajjat-al-Wada, "Which month (of the year) do you think is most sacred?" The people said, "This current month of ours (the month of Dhull-Hijja)." He said, "Which town (country) do you think is the most sacred?" They said, "This city of ours (Mecca)." He said, "Which day do you think is the most sacred?" The people said, "This day of ours." He then said, "Allah, the Blessed, the Supreme, has made your blood, your property and your honor as sacred as this day of yours in this town of yours, in this month of yours (and such protection cannot be slighted) except rightfully." He then said thrice, "Have I conveyed Allah's Message (to you)?" The people answered him each time saying, 'Yes." The Prophet added, 'May Allah be merciful to you (or, woe on you)! Do not revert to disbelief after me by cutting the necks of each other.'
Sahih Muslim 98. Narrated Tamim ad-Dari:
The Prophet of Allaah (may peace and blessings be upon him) observed: Al-Din is a name of sincerity and well wishing. Upon this we said: For whom? He replied: For Allaah, His Book, His Messenger and for the leaders and the general Muslims.
I can't look up the citations from the Sahih at-Tirmidhi because the only translations I can find go from hadith numbers 1-360 and another from 5000 to the end. I can't find the Masnud of Imam Ahmed bin Hanbal anywhere. The only references to this work appear to be repetitions of the Farewell Sermon text, and the one link that looked promising is in Arabic. Ahmed bin Hanbal was a real, and significant, Sunni scholar, however, so I'm sure there must be some actual collection they're referring to.
As you can see, only one of these sources actually makes a direct reference to the Farewell Sermon, and that one tells it as a brief call and response between Muhammad and his followers.
My guess is that this isn't mentioned in the citations I couldn't read either. I think it's most likely an example of Muslim glurge, something which I suppose must spread quite quickly among the many Western Muslims who are separated from the bulk of their religious texts by a pretty inpenetrable language barrier. I'm surprised though, by how often it gets repeated. I mean, Muhammad was quite adamant that anyone who put words in his mouth would get an everlasting arse-kicking for it (See Sahih Bukhari Volume 4, Book 56, Number 667)