Sahih Bukhari Volume 3, Book 28, Hadith Number 33.

Narated By Nafi : When Abdullah bin Umar set out for Mecca intending to perform Umra, at the time of afflictions, he said, “If I should be prevented from reaching the Kaba, then I would do the same as Allah’s Apostle did, so I assume the lhram for Umra as Allah’s Apostle assumed the Ihram for Umra in the year of Hudaibiya.”