Abu Dawud Book 27, Hadith Number 4031.
Chapter : Wearing silk.
Narated By N/A : Abu ‘Uthman al-Nahdi said: ‘Umar wrote to ‘Utbah b. Farqad that the Prophet (may peace be upon him) forbade (wearing) silk except so and so, and so-and-so, to the extent of two, three, or four fingers.