From Sahih Muslim (Hadith 5413):
Abu 'Uthman reported:While we were with 'Utba b. Farqad there came a letter of 'Umar (containing the instructions) that Allah's Messenger (ﷺ) had said: None should wear silk (with the exception of so much) but he will have nothing of it in the Hereafter. Abu 'Uthman said: To the extent of two fingers which are close to the thumb, and I was shown the (silk) borders of the Tayalisa mantle (which were about two fingers in breadth and I saw them)
Reference: Muslim 5413