Sahih Bukhari Volume 001, Book 003, Hadith Number 067.
Narated By 'Abdur Rahman bin Abi Bakra's father : Once the Prophet was riding his camel and a man was holding its rein. The Prophet asked, "What is the day today?" We kept quiet, thinking that he might give that day another name. He said, "Isn't it the day of Nahr (slaughtering of the animals of sacrifice)" We replied, "Yes." He further asked, "Which month is this?" We again kept quiet, thinking that he might give it another name. Then he said, "Isn't it the month of Dhul-Hijja?" We replied, "Yes." He said, "Verily! Your blood, property and honor are sacred to one another (i.e. Muslims) like the sanctity of this day of yours, in this month of yours and in this city of yours. It is incumbent upon those who are present to inform those who are absent because those who are absent might comprehend (what I have said) better than the present audience."