A letter from Umar reached us while we were in Adharbayjan with Utba bin Farqad, or in Sham. It read, "To proceed: The Messenger of Allah ﷺ forbade silk except for this much," indicating two fingers' width.Abu Uthman said, "We had no doubt that he meant the embroidered borders of garments."
حدثنا محمد بن المثنى وابن بشار واللفظ لابن المثنى قالا حدثنا محمد بن جعفر حدثنا شعبة عن قتادة قال سمعت
جاءنا كتاب عمر ونحن بأذربيجان مع عتبة بن فرقد أو بالشام أما بعد فإن رسول الله ﷺ نهى عن الحرير إلا هكذا إصبعين قال أبو عثمان فما عتمنا أنه يعني الأعلام
Both Bukhari and Muslim agreed on including the hadith of Abu Uthman, who said, "Umar wrote to us concerning silk, except for the width of two fingers." Abu Uthman did not hear this directly from Umar, since it was something Umar had written to him, and this serves as evidence for accepting authorization by correspondence as a valid means of transmission.