When ʿAbdullah bin ʿUmar set out for Mecca intending to perform Umra, at the time of afflictions, he said, "If I should be prevented from reaching the Kaʿba, then I would do the same as Messenger of Allah ﷺ did, so I assume the lhram for Umra as Messenger of Allah ﷺ assumed the Ihram for Umra in the year of Hudaibiya."  

عَنِ الْبَيْتِ صَنَعْتُ كَمَا صَنَعْنَا مَعَ رَسُولِ اللَّهِ ﷺ فَأَهَلَّ بِعُمْرَةٍ مِنْ أَجْلِ أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ أَهَلَّ بِعُمْرَةٍ عَامَ الْحُدَيْبِيَةِ  

That Ubaidullah bin ʿAbdullah and Salim bin ʿAbdullah informed him that they told Ibn ʿUmar when Ibn Az-Zubair was attacked by the army, saying "There is no harm for you if you did not perform Hajj this year. We are afraid that you may be prevented from reaching the Kaʿba." Ibn ʿUmar said "We set out with Messenger of Allah ﷺ and the non-believers of Quraish prevented us from reaching the Kaʿba, and so the Prophet ﷺ slaughtered his Hadi and got his head shaved." Ibn ʿUmar added, "I make you witnesses that I have made ʿUmra obligatory for me. And, Allah willing, I will go and then if the way to Kaʿba is clear, I will perform the Tawaf, but if I am prevented from going to the Kaʿba then I will do the same as the Prophet ﷺ did while I was in his company." Ibn ʿUmar then assumed Ihram for Umra from Dhul-Hulaifa and proceeded for a while and said, "The conditions of ʿUmra and Hajj are similar and I make you witnesses that I have made ʿUmra and Hajj obligatory for myself." So, he did not finish the Ihram till the day of Nahr (slaughtering) came, and he slaughtered his Hadi. He used to say, "I will not finish the Ihram till I perform the Tawaf, one Tawaf on the day of entering Mecca (i.e. of Safa and Marwa for both ʿUmra and Hajj).  

خَرَجْنَا مَعَ رَسُولِ اللَّهِ ﷺ فَحَالَ كُفَّارُ قُرَيْشٍ دُونَ الْبَيْتِ فَنَحَرَ النَّبِيُّ ﷺ هَدْيَهُ وَحَلَقَ رَأْسَهُ وَأُشْهِدُكُمْ أَنِّي قَدْ أَوْجَبْتُ الْعُمْرَةَ إِنْ شَاءَ اللَّهُ أَنْطَلِقُ فَإِنْ خُلِّيَ بَيْنِي وَبَيْنَ الْبَيْتِ طُفْتُ وَإِنْ حِيلَ بَيْنِي وَبَيْنَهُ فَعَلْتُ كَمَا فَعَلَ النَّبِيُّ ﷺ وَأَنَا مَعَهُ فَأَهَلَّ بِالْعُمْرَةِ مِنْ ذِي الْحُلَيْفَةِ ثُمَّ سَارَ سَاعَةً ثُمَّ قَالَ إِنَّمَا شَأْنُهُمَا وَاحِدٌ أُشْهِدُكُمْ أَنِّي قَدْ أَوْجَبْتُ حَجَّةً مَعَ عُمْرَتِي فَلَمْ يَحِلَّ مِنْهُمَا حَتَّى حَلَّ يَوْمَ النَّحْرِ وَأَهْدَى وَكَانَ يَقُولُ لاَ يَحِلُّ حَتَّى يَطُوفَ طَوَافًا وَاحِدًا يَوْمَ يَدْخُلُ مَكَّةَ  

Some of the sons of ʿAbdullah told him (i.e. ʿAbdullah) if he had stayed (and not performed Hajj that year).  

لَهُ لَوْ أَقَمْتَ بِهَذَا  

Messenger of Allah ﷺ was prevented from performing (ʿUmra) Therefore, he shaved his head and had sexual relations with his wives and slaughtered his Hadi and performed Umra in the following year.  

ابْنُ عَبَّاسٍ ؓ قَدْ أُحْصِرَ رَسُولُ اللَّهِ ﷺ فَحَلَقَ رَأْسَهُ وَجَامَعَ نِسَاءَهُ وَنَحَرَ هَدْيَهُ حَتَّى اعْتَمَرَ عَامًا قَابِلاً