Musnad Ahmad 32:1738

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Qatadah narrating from Anas ibn Malik, he said: "The Prophet, peace and blessings of Allah be upon him, granted a concession—or permitted—'Abd al-Rahman ibn 'Awf and al-Zubayr ibn al-'Awwam to wear silk, because of an itch that they both had."