Sahih Bukhari Volume 7, Book 72, Hadith Number 720.

Narrated By Abu ‘Uthman : While we were with ‘Utba. ‘Umar wrote to us: The Prophet said, “There is none who wears silk in this world except that he will wear nothing of it in the Hereafter.” ‘ Abu ‘Uthman pointed out with his middle and index fingers.