Nafi' (may Allah have mercy on him) said: Ibn Umar (may Allah be pleased with them both) set out for Hajj at the time when al-Hajjaj had reached Makkah to fight Ibn al-Zubayr (may Allah be pleased with them both). He was told: There is going to be fighting between the people, and we fear that they will prevent you. He said: For you, in the Messenger of Allah (peace and blessings be upon him) there is an excellent example [Surah al-Ahzab: 21]; I will do just as the Messenger of Allah (peace and blessings be upon him) did. I call you to witness that I have made Umrah binding upon myself. Then he set out until, when he climbed onto al-Bayda', he said: The matter of Hajj and Umrah is one and the same; I call you to witness that I have made Umrah binding along with Hajj. He also bought a sacrificial animal at Qudayd, and went on until he reached Makkah. He performed the tawaf of the House and of Safa and Marwah and did nothing more: he did not sacrifice, nor shave his head, nor cut his hair, and he did not come out of any restriction that had become forbidden to him, until, when the Day of Sacrifice came, he sacrificed and shaved his head. Then he considered that he had completed the tawaf for Hajj and Umrah with his first tawaf. Then he said: The Messenger of Allah (peace and blessings be upon him) did the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8782