Sahih Bukhari Volume 7, Book 72, Hadith Number 718.

Narrated By 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.