Musnad Ahmad 27:1767

I read to Abd al-Rahman: Malik, from Nafi’, that Abdullah ibn Umar went out to Makkah to perform Umrah during the time of the tribulation (fitnah). He said: "If I am prevented from reaching the House, we shall do as we did with the Messenger of Allah, may Allah bless him and grant him peace." Then he entered into the state of Ihram for Umrah, because the Prophet, may Allah bless him and grant him peace, entered into the state of Ihram for Umrah in the year of al-Hudaybiyyah.