صحيح البخاري

Narrated Abu `Uthman:

While we were at Adharbijan, `Umar wrote to us: 'Allah's Messenger (ﷺ) forbade wearing silk except this much. Then the Prophet (ﷺ) approximated his two fingers (index and middle fingers) (to illustrate that) to us.' Zuhair (the sub-narrator) raised up his middle and index fingers.