Narrated Nafi:

When Abdullah bin 'Umar set out for Mecca with the intentions performing 'Umra in the period of afflictions, he said, "If I should be prevented from reaching the Ka'ba, then I would do the same as we did while in the company of Allah's Apostle ." So, he assumed the Ihram for 'Umra since the Prophet had assumed the Ihram for 'Umra in the year of Al-Hudaibiya. Then 'Abdullah bin 'Umar thought about it and said, "The conditions for both Hajj and 'Umra are similar." He then turned towards his companions and said, "The conditions of both Hajj and 'Umra are similar and I make you witnesses that I have made the performance of Hajj obligatory for myself along with 'Umra." He then performed one Tawaf (between As-Safa and Al-Marwa) for both of them (i.e. Hajj and ('Umra) and considered that to be sufficient for him and offered a Hadi.

حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ ـ رضى الله عنهما ـ قَالَ حِينَ خَرَجَ إِلَى مَكَّةَ مُعْتَمِرًا فِي الْفِتْنَةِ إِنْ صُدِدْتُ عَنِ الْبَيْتِ صَنَعْنَا كَمَا صَنَعْنَا مَعَ رَسُولِ اللَّهِ ﷺ فَأَهَلَّ بِعُمْرَةٍ، مِنْ أَجْلِ أَنَّ النَّبِيَّ ﷺ كَانَ أَهَلَّ بِعُمْرَةٍ عَامَ الْحُدَيْبِيَةِ، ثُمَّ إِنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ نَظَرَ فِي أَمْرِهِ فَقَالَ مَا أَمْرُهُمَا إِلاَّ وَاحِدٌ. فَالْتَفَتَ إِلَى أَصْحَابِهِ فَقَالَ مَا أَمْرُهُمَا إِلاَّ وَاحِدٌ، أُشْهِدُكُمْ أَنِّي قَدْ أَوْجَبْتُ الْحَجَّ مَعَ الْعُمْرَةِ، ثُمَّ طَافَ لَهُمَا طَوَافًا وَاحِدًا، وَرَأَى أَنَّ ذَلِكَ مُجْزِيًا عَنْهُ، وَأَهْدَى.