Musnad Ahmad 32:1309

Asbat narrated to us, Sa'id narrated to us, from Qatadah, from Anas ibn Malik, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) granted a concession to Az-Zubayr ibn al-Awwam and Abd al-Rahman ibn Awf to wear silk while traveling, due to an itch that they both had."