Sahih Muslim Book 007, Hadith Number 2810.
Chapter : Permissibility of saying one has entered into Ihram like the Ihram of the other one or Abrogation of putting off Ihram and completing (Hajj and Umra).
Abu Musa (Allah be pleased with him) said: I came to the Messenger of Allah (may peace be upon him) as he was encamping at Batha. He said to me: Did you intend to perform Hajj? I said: Yes. He again said: With what intention have you entered into the state of Ihram (for Ifrad, Qiran or Tamattu'). I said: I pronounced Talbiya (I have entered into the state of Ihram) with that very aim with which the Apostle of Allah (may peace be upon him) is pronouncing Talbiya. He (the Holy Prophet) said; You have done well. Then circumambulate the House and run between al-Safa' and al-Marwa' and put off Ihram (as you have not brought the sacrificial animals along with you). So I circumambulated the House, and ran between al-Safa' and al-Marwa' and then came to a woman of the tribe of Qais and she rid my head of the lice. I again put on Ihram for Hajj and continued giving religious verdict (according to this practice) till during the Caliphate of Umar (Allah be pleased with him) when a person said to him: Abu Musa, or Abdullah b. Qais, exercise restraint in delivering some religious verdict of yours, for you do not know what has been introduced after you by the Commander of the Believers in the rites (of Hajj). Thereupon he said: O people, whom we gave the religious verdict (concerning putting off Ihram) they should wait, for the Commander of the Believers is about to come to you, and you should follow him. Umar (Allah be pleased with him) then came and I made a mention of it to him, whereupon he said: If we abide by the Book of Allah (we find) the Book of Allah has commanded us to complete the (.Hajj and 'Umra), and if we abide by the Sunnah of Allah's Messenger (may peace be upon him), we find that Allah's Messenger (may peace be upon him) did not put off Ihram till the sacrificial animal was brought to its end (till it was sacrificed).