Sahih Bukhari Volume 002, Book 026, Hadith Number 635.
Narated By Ibn Abbas : The people (of the Pre-lslamic Period) used to think that to perform 'Umra during the months of Hajj was one of the major sins on earth. And also used to consider the month of Safar as a forbidden (i.e. sacred) month and they used to say, "When the wounds of the camel's back heal up (after they return from Hajj) and the signs of those wounds vanish and the month of Safar passes away then (at that time) 'Umra is permissible for the one who wishes to perform it." In the morning of the 4th of Dhul-Hijja, the Prophet and his companions reached Mecca, assuming Ihram for Hajj and he ordered his companions to make their intentions of the Ihram for'Umra only (instead of Hajj) so they considered his order as something great and were puzzled, and said, "O Allah's Apostle! What kind (of finishing) of Ihram is allowed?" The Prophet replied, "Finish the Ihram completely like a non-Muhrim (you are allowed everything)."