Abdullah bin Abbas (may Allah be pleased with them both) narrates that I heard the Messenger of Allah (ﷺ) saying while delivering a sermon: "When a person in Ihram does not have shoes, he should wear socks, and when he does not find an izar (waist-wrapper), he should wear trousers."
Hadith Referenceمسند الإمام الشافعي / كتاب الحج / 841
Hadith Takhrijاخرجه البخاري، الحج، باب الحطبة ايام منى (1740) ومسلم، الحج، باب ما يباح للمحرم بحج أو عمرة ليسه ...... الخ (1178)۔