Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), while delivering a sermon at a place called al-Jabiyah, said that the Prophet (peace and blessings be upon him) "forbade wearing silk, but permitted silk to the width of one, two, three or four fingers."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4209