Hajj - The Sermon at `Arafaat (Ahadith 1496 - 1498)


Bismillah.
Volume 2, Book 26, Number 796:
Narrated Ibn Abbas (radiallaahu `anhu):
I heard the Prophet (sallallaahu `alayhi wasallam)  delivering a sermon at 'Arafat.

Volume 2, Book 26, Number 797:
Narrated Abu Bakra (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) delivered to us a sermon on the Day of Nahr. He said, "Do you know what is the day today?" We said, "Allah and His Apostle (sallallaahu `alayhi wasallam) know better." He remained silent till we thought that he might give that day another name. He said, "Isn't it the Day of Nahr?" We said, "It is." He further asked, "Which month is this?" We said, "Allah and His Apostle (sallallaahu `alayhi wasallam) know better." He remained silent till we thought that he might give it another name. He then said, "Isn't it the month of Dhul-Hijja?" We replied: "Yes! It is." He further asked, "What town is this?" We replied, "Allah and His Apostle (sallallaahu `alayhi wasallam) know it better." He remained silent till we thought that he might give it another name. He then said, "Isn't it the forbidden (Sacred) town (of Mecca)?" We said, "Yes. It is." He said, "No doubt, your blood and your properties are sacred to one another like the sanctity of this day of yours, in this month of yours, in this town of yours, till the day you meet your Lord. No doubt! Haven't I conveyed Allah's message to you? They said, "Yes." He said, "O Allah! Be witness. So it is incumbent upon those who are present to convey it (this information) to those who are absent because the informed one might comprehend it (what I have said) better than the present audience, who will convey it to him. Beware! Do not renegade (as) disbelievers after me by striking the necks (cutting the throats) of one another."

Volume 2, Book 26, Number 798:
Narrated Ibn 'Umar (radiallaahu `anhu):
At Mina, the Prophet (sallallaahu `alayhi wasallam) said, "Do you know what is the day today?" The people replied, "Allah and His Apostle (sallallaahu `alayhi wasallam) know it better." He said, "It is the forbidden (sacred) day. And do you know what town is this?" They replied, "Allah and His Apostle (sallallaahu `alayhi wasallam) know it better." He said, "This is the forbidden (Sacred) town (Mecca). And do you know which month is this?" The people replied, "Allah and His Apostle (sallallaahu `alayhi wasallam) know it better." He said, "This is the forbidden (sacred) month." The Prophet (sallallaahu `alayhi wasallam) added, "No doubt, Allah made your blood, your properties, and your honor sacred to one another like the sanctity of this day of yours in this month of yours in this town of yours." Narrated Ibn 'Umar (radiallaahu `anhu): On the Day of Nahr (10th of Dhul-Hijja), the Prophet (sallallaahu `alayhi wasallam) stood in between the Jamrat during his Hajj which he performed (as in the previous Hadith) and said, "This is the greatest Day (i.e. 10th of Dhul-Hijjah)." The Prophet (sallallaahu `alayhi wasallam) started saying repeatedly, "O Allah! Be Witness (I have conveyed Your Message)." He then bade the people farewell. The people said, "This is Hajjat-al-Wada)."

Another version of the sermon found at IslamQA:
“O people, listen to my words, for I do not know if I will meet you again after this year. O people, your blood, your wealth and your honour are sacred to you as the sanctity of this day of yours, in this month of yours, in this land of yours. Every practice of the jaahiliyyah is beneath my feet and the blood feuds of the jaahiliyyah are cancelled. The first claim of blood that I abolish is that of Ibn Rabee’ah ibn al-Haarith, who was suckled among the tribe of Bani Sa’d and was killed by Hudhayl. The ribaa of the jaahiliyyah is abolished, and the first ribaa that I abolish is that of ‘Abbaas ibn ‘Abd al-Muttalib. It is abolished completely.
Fear Allaah concerning women, for you have taken them on the security of Allaah and have made their bodies lawful to you by the words of Allaah.  Your right over them is that they should not allow anyone whom you dislike to sit on your bed. If they do that, then hit them, but not severely. Their rights over you are that you should be kind to them and clothe them properly.
I have left you with something which, if you adhere to it, you will not go astray after I am gone: the Book of Allaah. If you were asked about me, what would you say?” They said: “We would bear witness that you have conveyed (the message), fulfilled (the trust) and advised us sincerely.” Then he pointed with his index finger towards the sky and then towards the people, and said, “O Allaah, bear witness, O Allaah bear witness,” three times.
When Allaah perfected this religion and its basic principles had been established, Allaah revealed to him in ‘Arafaah:
“This day, I have perfected your religion for you, completed My Favour upon you, and have chosen for you Islam as your religion
[al-Maa'idah 5:3 – interpretation of the meaning] 
Here's another longer version. Don't know about its authenticity, though.
There's a lot of lessons to be derived from this sermon, and you'll find a lot of material on it online and in books etc. I'll just make one point here. People who are more knowledgeable than others can revive this Sunnah during Hajj - this Sunnah of preaching at `Arafaat. Advise your fellow hujjaaj (pilgrims) and admonish them. It doesn't have to be a sermon like that of the Prophet (sallallaahu `alayhi wasallam), but it can be a reminder or two taken from his teachings. It's doable, right?
Did you know? This Hajj is called Hujjat al-Wadaa’ (the Farewell Pilgrimage) because in it the Messenger (peace and blessings of Allaah be upon him) bade farewell to the people, and he did not perform Hajj after that.