Sunan Abi Dawud 4042
سنن أبي داود#4042SahihNarrated Abu 'Uthman al-Nahdi:'Umar wrote to 'Utbah b. Farqad that the Prophet (ﷺ) forbade (wearing) silk except so-and-so, and so-and-so, to the extent of two, three, or four fingers
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Bukhari (5829) Sahih Muslim (2069)
In-book reference: 34:23