Sahih Bukhari Volume 003, Book 027, Hadith Number 021.
Narated By Abu Musa Al-Ashari : I came to the Prophet at Al-Batha' while his camel was kneeling down and he asked me, "Have you intended to perform the Hajj?" I replied in the affirmative. He asked me, 'With what intention have you assumed Ihram?" I replied, "I have assumed Ihram with the same intention as that of the Prophet. He said, "You have done well. Perform the Tawaf of the Ka'ba and (the Sai) between As-safa and Al-Marwa and then finish the Ihram." So, I performed the Tawaf around the Ka'ba and the Sai) between As-Safa and Al-Marwa and then went to a woman of the tribe of Qais who cleaned my head from lice. Later I assumed the Ihram for Hajj. I used to give the verdict of doing the same till the caliphate of 'Umar who said, "If you follow the Holy Book then it orders you to remain in the state of Ihram till you finish from Hajj, if you follow the Prophet then he did not finish his Ihram till the Hadi (sacrifice) had reached its place of slaughtering (Hajj-al-Qiran)."