Sahih Bukhari Volume 7, Book 72, Hadith Number 719.

Narrated By Abu ‘Uthman : While we were at Adharbijan, ‘Umar wrote to us: ‘Allah’s Apostle forbade wearing silk except this much. Then the Prophet approximated his two fingers (index and middle fingers) (to illustrate that) to us.’ Zuhair (the sub-narrator) raised up his middle and index fingers.