Sahih Muslim
The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wal-Zinah)
Book Number: 024 / Hadith Number: 5156
Suwaid b. Ghafala said: 'Umar addressed us at a place known as Jabiya (Syria) and he said: Allah's Apostle (may peace be upon him) forbade us the wearing of silk but to the extent of two or three fingers or four fingers.