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.

حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا عَاصِمٌ، عَنْ أَبِي عُثْمَانَ، قَالَ كَتَبَ إِلَيْنَا عُمَرُ وَنَحْنُ بِأَذْرَبِيجَانَ أَنَّ النَّبِيَّ صلى الله عليه وسلم نَهَى عَنْ لُبْسِ الْحَرِيرِ إِلاَّ هَكَذَا، وَصَفَّ لَنَا النَّبِيُّ صلى الله عليه وسلم إِصْبَعَيْهِ‏.‏ وَرَفَعَ زُهَيْرٌ الْوُسْطَى وَالسَّبَّابَةَ‏.‏