Musnad Ahmad 27:1808

Ibn Numayr narrated to us, Ubaydullah narrated to us, from Nafi’, from Ibn Umar, who said: "If I am prevented from reaching the House (the Kaaba), we shall do as we did with the Messenger of Allah, peace and blessings of Allah be upon him, when the disbelievers of Quraysh prevented him from reaching the House; so he shaved (his head) and returned." And I call you to witness that I have made an Umrah obligatory upon myself. Then he mentioned the rest of the hadith.