From Sahih Bukhari (Hadith 5828):
Narrated Aba `Uthman An-Nahdi:While we were with `Utba bin Farqad at Adharbijan, there came `Umar's letter indicating that Allah's Apostle had forbidden the use of silk except this much, then he pointed with his index and middle fingers. To our knowledge, by that he meant embroidery
Reference: Bukhari 5828