Sahih Bukhari, Chapter: Pilgrims Prevented from Completing the Pilgrimage

When Abdullah bin Umar set out for Mecca with the intentions performing 'Umra in the period of afflictions, he said, "If I should be prevented from reaching the Kaba, then I would do the same as we did while in the company of Allah's Apostle." So, he assumed the Ihram for Umra since the Prophet had assumed the Ihram for Umra in the year of Al-Hudaibiya. Then Abdullah bin Umar thought about it and said, "The conditions for both Hajj and 'Umra are similar." He then turned towards his companions and said, "The conditions of both Hajj and Umra are similar and I make you witnesses that I have made the performance of Hajj obligatory for myself along with Umra." He then performed one Tawaf (between As-Safa and Al-Marwa) for both of them (i.e. Hajj and Umra) and considered that to be sufficient for him and offered a Hadi.