The ruling on whoever forgot to shave his Head or cut his Hair
Question :
What is the ruling on whoever forgot to shave his head or cut it for 'Umrah, and he wore seamed garments and then remembered that he did not cut his hair or shave his head?
Answer:
Whoever forgot to shave his head or cut his hair for 'Umrah, and he already performed Tawaf and Sa'y, and then he wore (normal) clothes before shaving his head or cutting his hair, he must remove his (normal) clothes when he remembers. Then he must shave his head or cut his hair, and then put back on his (normal) clothes. If he cut his hair or shaved his head while wearing his (normal) clothing, due to ignorance or forgetfulness on his part, there is no sin on him and that suffices him. There is no need for him to repeat cutting his hair or shaving his head. However, whenever he becomes aware (of what he should be doing), then it is obligatory on him to remove the normal clothes so that he can shave his head or cut his hair while he is in Ihram.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 4 Pages 208-209