Abdullah bin Umar set out for Mecca to perform umrah during the time of civil strife. He said, "If I am prevented from reaching the House, I will do as we did with the Messenger of Allah ﷺ." So he entered the state of ihram for umrah, because the Prophet ﷺ had entered ihram for umrah in the year of Hudaybiyyah.
قال قرأت على عبد الرحمن مالك
أن عبد الله بن عمر خرج إلى مكة معتمرا في الفتنة فقال إن صددت عن البيت صنعنا كما صنعنا مع رسول الله ﷺ فأهل بعمرة من أجل أن النبي ﷺ أهل بعمرة عام الحديبية
Its isnad is sahih. It is a longer version of 5298 with this same isnad. We noted there that it appears in Muwatta 1:329-330 at greater length, so this too is an abridgment of what is in the Muwatta. It has also appeared at length several times through a route other than Malik, the most recent being 5322. See also 6067.