Musnad Abu Ya'la al-Mawsili 153:86

'Abdullah ibn Muhammad ibn Asma' narrated to us, Juwayriyah narrated to us, from Nafi', that 'Ubaydullah ibn 'Abdullah and Salim ibn 'Abdullah informed him that they spoke to 'Abdullah [ibn 'Umar] during the nights when the army descended upon Ibn al-Zubayr before he was killed. They said, "It will not harm you if you do not perform Hajj this year, for we fear that you will be prevented from reaching the House." He said, "We went out with the Messenger of Allah (may Allah bless him and grant him peace), and the disbelievers of Quraysh stood between us and the House. So the Messenger of Allah (may Allah bless him and grant him peace) slaughtered his sacrificial animal and shaved his head. I call you to witness that I have made an 'Umrah obligatory, if Allah wills. I will set out; if I am allowed to reach the House, I will perform Tawaf, and if I am prevented from it, I will do as the Messenger of Allah (may Allah bless him and grant him peace) did, and I am with him." So he entered into the state of Ihram for 'Umrah at Dhu al-Hulayfah. Then he traveled for a while and said, "Their matter is but one, and I call you to witness that I have made a Hajj obligatory along with my 'Umrah." He did not exit the state of Ihram for either of them until he exited on the Day of Sacrifice. He used to say, "Whoever combines Hajj and 'Umrah and enters into the state of Ihram for both, he does not exit the state of Ihram until he exits from both of them together on the Day of Sacrifice. He then performs one Tawaf for both at the House, and [walks] between al-Safa and al-Marwah on the day he enters Makkah."