Praise be to Allah.
Shaykh Ibn ‘Uthaymeen said:
There is nothing wrong with wearing a belt over the izar (waist wrapper).
With regard to the questioner saying that it is stitched, this comment is based on a misunderstanding, because some of the common folk think that what is meant by it is anything that has stitches in it, but that is not correct. Rather what the scholars mean by wearing things that are stitched is that which is made to fit the limbs and is worn in the manner in which such garments are usually worn, such as shirts, pants and T-shirts, etc. The scholars did not mean that which has stitches in it. Hence if a person were to enter ihraam in a patched rida (upper garment) and a patched izar, there would be nothing wrong with that, even though part of it is stitched to another part.