كتاب الحج
15  
The Book of Pilgrimage
(1)
Chapter: What one who has entered Ihram for Hajj or Umrah is permitted to wear, and what is not permissible, and perfume is forbidden for him
(1)
باب مَا يُبَاحُ لِلْمُحْرِمِ بِحَجٍّ أَوْ عُمْرَةٍ وَمَا لاَ يُبَاحُ وَبَيَانِ تَحْرِيمِ الطِّيبِ عَلَيْهِ
Sahih Muslim 1179

Jabir (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying:

He who does not find shoes to wear may wear socks, and he who does not find lower garment to wear may put on trousers.
وَحَدَّثَنَا أَحْمَدُ بْنُ عَبْدِ اللَّهِ بْنِ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا أَبُو الزُّبَيْرِ، عَنْ جَابِرٍ، - رضى الله عنه - قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ مَنْ لَمْ يَجِدْ نَعْلَيْنِ فَلْيَلْبَسْ خُفَّيْنِ وَمَنْ لَمْ يَجِدْ إِزَارًا فَلْيَلْبَسْ سَرَاوِيلَ ‏"‏ ‏.‏
Reference : Sahih Muslim 1179
In-book reference : Book 15, Hadith 7
USC-MSA web (English) reference : Book 7, Hadith 2653
  (deprecated numbering scheme)