Reference: The Verdicts of the Permanent Committee – Volume 11, Page 171, Fatwa No. 8593
Question: Why does the pilgrim wear those clothes during Hajj?
Response: All praise be to Allaah.
Allaah commanded us, on the tongue of His Messenger Muhammad (praise and peace of Allaah be upon him) to wear the izaar (lower garment) and ridaa’ (upper garment) during Hajj and ‘Umrah for some wisdom that is known to Him. So we must obey (Him) to hope to earn the reward, whether we know the wisdom or not. Some of what the scholars have said concerning that is that it is a reminder on how the people will be on the Day of Gathering and Raising, the Day of Resurrection, to make the pilgrim feel humble and that rich and poor are equal. We ask Allaah to give us and you success, make us correct and to remain steadfast in adhering to the truth until we meet Him.
And with Allaah lies All the success. And may Allaah praise and have peace upon our Prophet Muhammad his family and his companions.