Nafi' (may Allah have mercy on him) said: Abdullah bin Umar (may Allah be pleased with him) set out for Umrah during the time of the fitnah (civil strife) and said: If I am prevented from the House, then we will do as we did with the Messenger of Allah (peace and blessings be upon him). Abdullah bin Umar (may Allah be pleased with him) set out, having entered ihram for Umrah, and travelled until he climbed onto al-Bayda'. Then he turned to his companions and said: The matter of these two (Hajj and Umrah) is one and the same; I call you to witness that I have made Hajj obligatory upon myself along with Umrah. Then Abdullah bin Umar (may Allah be pleased with him) came to the House and performed tawaf, and the sa'y between Safa and Marwah, seven circuits each, and did no more than that. He held that this would suffice him, and he offered a sacrifice.
(b) In the narration of Ibn Bukayr: He entered ihram for Umrah because the Messenger of Allah (peace and blessings be upon him) had entered ihram for Umrah in the year of al-Hudaybiyah.
(c) Imam al-Shafi'i (may Allah have mercy on him) said: If we are prevented from the House, we will do as the Messenger of Allah (peace and blessings be upon him) did, that is, we will exit ihram just as we exited ihram with the Messenger of Allah (peace and blessings be upon him) in the year of al-Hudaybiyah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10075