Hadith 4323
وَعَنْ عُمَرَ رَضِيَ اللَّهُ عَنْهُ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ نَهَى عَنْ لُبْسِ الْحَرِيرِ إِلَّا هَكَذَا وَرَفَعَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إصبعيه: الْوُسْطَى والسبابة وضمهما
‘Umar said the Prophet forbade wearing silk except to this extent, which was indicated by God's messenger raising his middle and forefinger and joining them. Bukhari and Muslim.