Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this same chain of narration, it is narrated that Sayyiduna Ibn Umar (may Allah be pleased with them both) set out towards Makkah to perform Umrah during the time of turmoil (war), and he said: If I am prevented from reaching the House of Allah, then we will do as the Messenger of Allah (peace and blessings be upon him) did. Then he proclaimed the Talbiyah for Umrah for the reason that in the year of Hudaybiyyah, the Prophet (peace and blessings be upon him) had proclaimed the Talbiyah for Umrah. Then Abdullah bin Umar (may Allah be pleased with them both) reflected upon his situation and said: Both (Umrah and Hajj) have the same ruling; I make you witnesses that I have made Hajj obligatory upon myself along with Umrah. Then he performed one Tawaf and considered it sufficient, and offered a sacrifice.