Bukhari

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 Apostle did, so I assume the lhram for Umra as Allah's Apostle assumed the Ihram for Umra in the year of Hudaibiya."

And Allah the Exalted said: "But if you are prevented, then (offer) whatever sacrificial animal is available, and do not shave your heads (i.e., do not exit the state of Ihram) until the sacrificial animal reaches its place (i.e., until it is slaughtered in Mecca)." And Ata bin Abi Rabah, may Allah have mercy on him, said that whatever prevents (a person), the same ruling applies to it.