Musnad Ahmad 32:1305

Muhammad ibn Bakr narrated to us, Sa'id informed us, from Qatadah, from Anas ibn Malik, that he narrated to them: "The Prophet, peace and blessings of Allah be upon him, granted a concession to Abd al-Rahman ibn Awf and al-Zubayr ibn al-Awwam to wear silk shirts while on a journey, because of an itch they were suffering from."