بُلُوغُ الْمَرَامِ
Bulugh al-Maram 526
The Book of Prayer
وَعَنْ عُمَرَ - رضى الله عنه - قَالَ: { نَهَى اَلنَّبِيُّ - صلى الله عليه وسلم -عَنْ لُبْسِ اَلْحَرِيرِ إِلَّا مَوْضِعَ إِصْبَعَيْنِ, أَوْ ثَلَاثٍ, أَوْ أَرْبَعٍ } مُتَّفَقٌ عَلَيْهِ, وَاللَّفْظُ لِمُسْلِمٍ [1] .
Narrated 'Umar (RA): Allah's Messenger (ﷺ) forbade the wearing of silk except the space of two or three or four fingers. [Agreed upon, and the wording is from Muslim].
Sunnah.com reference: Book 2, Hadith 457