From Sahih Bukhari (Hadith 1806):
Narrated 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 Ka`ba, then I would do the same as Allah's Messenger (ﷺ) did, so I assume the Ihram for Umra as Allah's Messenger (ﷺ) assumed the Ihram for Umra in the year of Hudaibiya
Reference: Bukhari 1806