Musnad Ahmad 27:148

Sufyan narrated to us, Ayyub ibn Musa narrated to us, from Nafi’: Ibn Umar went out intending to perform Umrah, and they informed him that there was a situation in Mecca. He said: "I will enter into the state of Ihram for Umrah, and if I am detained, I will do as the Messenger of Allah (ﷺ) did." So he entered into the state of Ihram for Umrah. When he had traveled a little and was at al-Bayda, he said: "The way of Umrah is none other than the way of Hajj. I make Hajj obligatory." And he said: "I call you to witness that I have made Hajj obligatory, for the way of Hajj is the way of Umrah." He arrived in Mecca, performed Tawaf around the House seven times, and between al-Safa and al-Marwa seven times, and said: "This is how I saw the Messenger of Allah (ﷺ) do." He came to Qadid, bought a sacrificial animal, and drove it along with him.