Indeed, silk is worn by him who has no share (in the Hereafter)
Authentic hadithعن عمر بن الخطاب رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم :"لا تلبسوا الحرير فإن من لبسه في الدنيا لم يلبسه في الآخرة". وفي رواية: «إِنَّمَا يَلْبَسُ الحَرِيرَ مَنْ لا خَلَاقَ له». وفي رواية للبخاري: «مَنْ لا خَلَاقَ له في الآخرةِ».
‘Umar ibn al-Khattāb (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Indeed, silk is worn by him who has no share." According to the version of Al-Bukhāri: "... who has no share in the Hereafter."
Explanation
The Prophet (may Allah's peace and blessings be upon him) said that men who wear silk are those who will have no share (of good) in the Hereafter. This statement is a severe threat, because wearing silk is allowed for women only, and it is also the clothing of the people of Paradise. Only the people of arrogance, pride, and vanity wear silk in the worldly life. For this reason, the Prophet (may Allah's peace and blessings be upon him) prohibited men from wearing it. Although this prohibition is specific to natural silk, men should avoid wearing synthetic silk as well given its softness. But it is not prohibited to wear it according to a Fatwa by the Standing Committee (KSA).
Source · Al-Bukhari and Muslim with its two versions
From the Hadith Encyclopedia (hadeethenc.com).