All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is impermissible for a person in a state of Ihraam to wear stitched clothing which is clothing that is tailored to fit the body parts such as shirts, pants, and cloaks. It does not mean anything that has thread or is sewn as some people mistakenly believe.
This is based on the Hadeeth of the Prophet, sallallaahu ‘alayhi wa sallam, where he said: "A person in a state of Ihraam should wear neither a Qamees (i.e. shirt) nor turban." [Al-Bukhari and Muslim]
The wisdom behind this, and Allaah Knows best, is that this style urges a person to maintain a state of Khushoo‘, humbleness and submissiveness to Allaah The Almighty. It also reminds one of the Hereafter because, in such a state, he relinquishes the luxury and apparent adornment of the worldly life.
Moreover, this dress which is worn by all pilgrims stands for the unity of Muslims and equality between them. Hence, there is no room for anyone to boast or mock the dress of another. If the matter was left for the people to choose, the rich and poor would be distinguished from one another as would the rulers and subjects.
Furthermore, the clothing of Ihraam inspires a pilgrim with the spirit of Ihraam, so he indulges in intensified supplication and Thikr and guards himself against prohibitions and forbiddances.