Language:
كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(91)
Chapter: The wearing of silk in war
(91)
باب الْحَرِيرِ فِي الْحَرْبِ
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.‏
Reference : Sahih al-Bukhari 2922
In-book reference : Book 56, Hadith 135
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 172
  (deprecated numbering scheme)