Musnad Ahmad 32:348

Hajjaj narrated to us, Shu'bah narrated to me, from Qatadah, from Anas bin Malik, who said: "The Prophet, peace and blessings of Allah be upon him, granted a concession—or permitted—'Abd al-Rahman bin 'Awf and al-Zubayr bin al-'Awwam to wear silk because of an itch they both had."