Imam Malik's Muwatta, Chapter: Hajj

Yahya related to me from Malik from Nafi that when Abdullah ibn Umar set out for Makka during the troubles (between al-Hajjaj ibn Yusuf and Zubair ibn al-Awwam) he said, "If I am blocked from going to the House we shall do what we did when we were with the Messenger of Allah, may Allah bless him and grant him peace," and he went into ihram for umra, because that was what the Messenger of Allah, may Allah bless him and grant him peace, did in the year of al-Hudaybiya. But afterwards, he reconsidered his position and said, "It is the same either way." After that he turned to his companions and said, "It is the same either way. I call you to witness that I have decided in favour of hajj and umra together." He then got through to the House (without being stopped) and did one set of tawaf, which he considered to be enough for himself, and sacrificed an animal. Malik said, "This is what we go by if someone is hindered by an enemy, as the Prophet, may Allah bless him and grant him peace, and his companions were. If some one is hindered by anything other than an enemy, he is only freed from ihram by tawaf of the House."