Musnad Ahmad 181
It was narrated that ‘Abdullah, the freed slave of Asma’ bint Abi Bakr, said:
Asma’ sent me to 'Abdullah bin 'Umar, and she said: I have heard that you regard three things as haram: borders on garments, saddle cloths made of bright red cloth, and fasting the whole of Rajab. 'Abdullah said to me: As for what you have mentioned about Rajab, what about one who fasts continually? As for what you said about borders on garments, I heard 'Umar bin al-Khattab say: I heard the Messenger of Allah ﷺ say: “Whoever wears silk in this world will not wear it in the Hereafter.`
حَدَّثَنَا يَحْيَى، عَنْ عَبْدِ الْمَلِكِ، حَدَّثَنَا عَبْدُ اللَّهِ، مَوْلَى أَسْمَاءَ قَالَ أَرْسَلَتْنِي أَسْمَاءُ إِلَى ابْنِ عُمَرَ أَنَّهُ بَلَغَهَا أَنَّكَ تُحَرِّمُ أَشْيَاءَ ثَلَاثَةً الْعَلَمَ فِي الثَّوْبِ وَمِيثَرَةَ الْأُرْجُوَانِ وَصَوْمَ رَجَبٍ كُلِّهِ فَقَالَ أَمَّا مَا ذَكَرْتَ مِنْ صَوْمِ رَجَبٍ فَكَيْفَ بِمَنْ يَصُومُ الْأَبَدَ وَأَمَّا مَا ذَكَرْتَ مِنْ الْعَلَمِ فِي الثَّوْبِ فَإِنِّي سَمِعْتُ عُمَرَ رَضِيَ اللَّهُ عَنْهُ يَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ مَنْ لَبِسَ الْحَرِيرَ فِي الدُّنْيَا لَمْ يَلْبَسْهُ فِي الْآخِرَةِ‏.‏