Sahih al-Bukhari 2922
Narrated Anas:
(Wearing of silk) was allowed to them (i.e. `AbdurRahman and Az-Zubair) because of the itching they 
suffered from.
حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، سَمِعْتُ قَتَادَةَ، عَنْ أَنَسٍ، رَخَّصَ أَوْ رُخِّصَ لِحِكَّةٍ بِهِمَا‏.‏