Musnad Ahmad 92
It was narrated that Abu 'Uthman said:
A letter came to us from ‘Umar when we were in Azerbaijan (in which it said): O 'Utbah bin Farqad, beware of luxury, the clothing of the mushrikeen and wearing silk, for the Messenger of Allah (ﷺ) forbade us to wear silk and said, “Except this much,` and the Messenger of Allah (ﷺ) held up two fingers to us.
حَدَّثَنَا حَسَنُ بْنُ مُوسَى، قَالَ حَدَّثَنَا زُهَيْرٌ، قَالَ حَدَّثَنَا عَاصِمٌ الْأَحْوَلُ، عَنْ أَبِي عُثْمَانَ، قَالَ جَاءَنَا كِتَابُ عُمَرَ رَضِيَ اللَّهُ عَنْهُ وَنَحْنُ بِأَذْرَبِيجَانَ يَا عُتْبَةَ بْنَ فَرْقَدٍ وَإِيَّاكُمْ وَالتَّنَعُّمَ وَزِيَّ أَهْلِ الشِّرْكِ وَلَبُوسَ الْحَرِيرِ فَإِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ نَهَانَا عَنْ لَبُوسِ الْحَرِيرِ وَقَالَ إِلَّا هَكَذَا وَرَفَعَ لَنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِصْبَعَيْهِ‏.‏