Abu Uthman al-Nahdi said: A letter from Sayyiduna Umar (may Allah be pleased with him) came to us while we were with Sayyiduna Utbah bin Farqad (may Allah be pleased with him) in Azerbaijan, saying that the Prophet (peace and blessings be upon him) "permitted wearing silk only to the extent of two fingers, that is, as a border marking."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6081