Sahih Bukhari Volume 002, Book 026, Hadith Number 623.
Narated By Anas bin Malik : Allah's Apostle offered four Rakat of Zuhr prayer at Medina and we were in his company, and two Rakat of the Asr prayer at Dhul-Hulaifa and then passed the night there till it was dawn; then he rode, and when he reached Al-Baida', he praised and glorified Allah and said Takbir (i.e. Alhamdu-lillah and Subhanallah(1) and Allahu-Akbar). Then he and the people along with him recited Talbiya with the intention of performing Hajj and Umra. When we reached (Mecca) he ordered us to finish the lhram (after performing the Umra) (only those who had no Hadi (animal for sacrifice) with them were asked to do so) till the day of Tarwiya that is 8th Dhul-Hijja when they assumed Ihram for Hajj. The Prophet sacrificed many camels (slaughtering them) with his own hands while standing. While Allah's Apostle was in Medina he sacrificed two horned rams black and white in color in the Name of Allah."