Umar wrote to Utba bin Farqad that the Prophet ﷺ forbade silk except for what was this much and this much, two fingers, three, and four.
حدثنا موسى بن إسمعيل حدثنا حماد حدثنا عاصم الأحول
كتب عمر إلى عتبة بن فرقد أن النبي ﷺ نهى عن الحرير إلا ما كان هكذا وهكذا أصبعين وثلاثة وأربعة
Bukhari narrated it in the Book of Dress, Chapter on wearing silk for men and how much of it is permissible, no. 5829, and Muslim in the Book of Dress, Chapter on the prohibition of wearing silk and similar things for men, no. 2069, from the hadith of Asim Ahwal, through him.