Musnad Ahmad 27:716

Yahya narrated to us, from Ubaydullah, Nafi’ informed me that Abdullah ibn Abdullah and Salim ibn Abdullah spoke to Abdullah [ibn Umar] when Al-Hajjaj descended to fight Ibn al-Zubayr. They said: "It will not harm you if you do not perform Hajj this year, for we fear that there will be fighting among the people, and that you will be prevented from reaching the House." He said: "If I am prevented from it, I will do as the Messenger of Allah (ﷺ) did, and I was with him when the disbelievers of Quraysh prevented him from the House. I call you to witness that I have made an Umrah obligatory upon myself. If my path is cleared, I will complete my Umrah, and if I am prevented from it, I will do as the Messenger of Allah (ﷺ) did, and I was with him." Then he set out until he reached Dhu al-Hulayfah, where he entered into the state of Ihram for Umrah. Then he recited: "There has certainly been for you in the Messenger of Allah an excellent pattern" (Al-Ahzab: 21). Then he traveled until, when he was at Zahral-Bayda, he said: "Their matter is but one. If I am prevented from the Umrah, I will be prevented from the Hajj. I call you to witness that I have made a Hajj obligatory upon myself along with my Umrah." Then he proceeded until he bought a sacrificial animal at Qudayd. Then he performed one Tawaf of the House for both of them, and [one Sa'i] between Al-Safa and Al-Marwah, and he remained in that state until the Day of Sacrifice.