cutting them so that they come lower than the ankles.

Sunan an-Nasa'i · Book 24 · Hadith 2680

أَخْبَرَنَا 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 (ﷺ) said "If a Muhrim cannot find sandals then let him wear Khuffs, and cut them, so that they come lower than the ankles.''