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 1178 a

Ibn 'Abbas (Allah be pleased with both of them) reported:

I heard Allah's Messenger (ﷺ) say as he was delivering an address: So far as the trousers are concerned, one who does not find lower garment, he may wear them; as also socks, he may wear them who does not find shoes. It concerns the Muhrim.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَأَبُو الرَّبِيعِ الزَّهْرَانِيُّ، وَقُتَيْبَةُ بْنُ سَعِيدٍ، جَمِيعًا عَنْ حَمَّادٍ، - قَالَ يَحْيَى أَخْبَرَنَا حَمَّادُ بْنُ زَيْدٍ، - عَنْ عَمْرٍو، عَنْ جَابِرِ بْنِ زَيْدٍ، عَنِ ابْنِ عَبَّاسٍ، - رضى الله عنهما - قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم وَهُوَ يَخْطُبُ يَقُولُ ‏ "‏ السَّرَاوِيلُ لِمَنْ لَمْ يَجِدِ الإِزَارَ وَالْخُفَّانِ لِمَنْ لَمْ يَجِدِ النَّعْلَيْنِ ‏"‏ ‏.‏ يَعْنِي الْمُحْرِمَ ‏.‏
Reference : Sahih Muslim 1178a
In-book reference : Book 15, Hadith 4
USC-MSA web (English) reference : Book 7, Hadith 2650
  (deprecated numbering scheme)