Mishkat al-Masabih 10 The Rites of Pilgrimage كتاب المناسك 169

وَعَنِ ابْنِ عَبَّاسٍ قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَخْطُبُ وَهُوَ يَقُولُ: «إِذَا لَمْ يَجِدِ الْمُحْرِمُ نَعْلَيْنِ لَبَسَ خُفَّيْنِ وَإِذَا لَمْ يَجِدْ إِزَارًا لَبَسَ سَرَاوِيل»

Ibn ‘Abbas told of hearing God’s messenger saying in the course of an address, “When one who is on pilgrimage cannot get sandals he may wear shoes, and when he cannot get a lower garment he may wear trousers,” Bukhari and Muslim.