It is narrated from Abu Bakrah (may Allah be pleased with him) that he (peace and blessings be upon him) delivered a sermon at Mina and said: "Do you know what day this is?" We said: Allah and His Messenger (peace and blessings be upon him) know best. He (peace and blessings be upon him) remained silent, and we thought that he (peace and blessings be upon him) would give it another name. Then he said: "Is today not the Day of Sacrifice?" We said: Yes! Then he asked: "Which city is this?" We said: Allah and His Messenger (peace and blessings be upon him) know best. He said: "Is this not the sacred city?" We said: Of course, O Messenger of Allah (peace and blessings be upon him)! Then he (peace and blessings be upon him) said: "Your blood, your wealth and your honour are as sacred as the sanctity of this city and this day. Have I conveyed (the message) to you?" We said: Yes! Then he said: 'Allahumma ishhad' "O Allah, bear witness. Let the one who is present convey this to the one who is absent, for many a person to whom it is conveyed remembers it better than the one who conveys it." Then he said: "Do not revert to disbelief after me, killing one another."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15848