Sahih Bukhari Volume 002, Book 020, Hadith Number 191.

Narated By Ibn Abbas : The Prophet and his companions reached Mecca in the morning of the 4th Dhul-Hijja reciting Talbiya (O Allah! We are obedient to your orders, we respond 4 to your call)... intending to perform Hajj. The Prophet ordered his companions to assume the lhram for Umra instead of Hajj, excepting those who had Hadi (sacrifice) with them.