Sunan an-Nasai 17421
In-book no 2685أَخْبَرَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا هُشَيْمٌ، قَالَ أَنْبَأَنَا ابْنُ عَوْنٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، رضى الله عنهما عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا لَمْ يَجِدِ الْمُحْرِمُ النَّعْلَيْنِ فَلْيَلْبَسِ الْخُفَّيْنِ وَلْيَقْطَعْهُمَا أَسْفَلَ مِنَ الْكَعْبَيْنِ " .
It was narrated from Ibn 'Umar: That the Prophet said "If a Muhrim cannot find sandals then let him wear Khuffs, and cut them, so that they come lower than the ankles.''