Nafi reported,

Ubaydullah bin Abdullah and Salim bin Abdullah reported that they both spoke with Abdullah bin Umar during the nights when the army laid siege against Ibn Zubayr. They said to him, "It would do you no harm not to perform Hajj this year. We fear that you may be prevented from reaching the House."He replied, "We set out with the Messenger of Allah ﷺ, and the disbelievers of Quraysh stood between us and the House. So the Prophet ﷺ sacrificed his offering and shaved his head. I call you to witness that I have made Umrah obligatory upon myself. God willing, I will set out, and if the way to the House is left open to me, I will perform tawaf. But if I am prevented from reaching it, I will do as the Prophet ﷺ did when I was with him."Then he entered ihram for Umrah from Dhul Hulayfah. After traveling for a while, he said, "The two are really one matter. I call you to witness that I have made Hajj obligatory upon myself together with my Umrah."He did not exit ihram from either of them until he exited on the Day of Sacrifice, and he offered a sacrificial animal. He used to say that one does not exit ihram until performing a single tawaf on the day he enters Makkah.

عن نافع

أن عبيد الله بن عبد الله، وسالم بن عبد الله أخبراه أنهما كلما عبد الله بن عمر رضي الله عنهما ليالي نزل الجيش بابن الزبير، فقالا: لا يضرك أن لا تحج العام، وإنا نخاف أن يحال بينك وبين البيت! فقال: خرجنا مع رسول الله ﷺ فحال كفار قريش دون البيت، فنحر النبي ﷺ هديه وحلق رأسه، وأشهدكم أني قد أوجبت العمرة إن شاء الله أنطلق فإن خلي بيني وبين البيت طفت، وإن حيل بيني وبينه فعلت كما فعل النبي ﷺ وأنا معه، فأهل بالعمرة من ذي الحليفة، ثم سار ساعة ثم قال: إنما شأنهما واحد، أشهدكم أني قد أوجبت حجة مع عمرتي فلم يحل منهما حتى حل يوم النحر وأهدى. وكان يقول: لا يحل حتى يطوف طوافا واحدا يوم يدخل مكة.