Musannaf Ibn Abi Shayba 3:280

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Khalid al-Hadhdha’, from al-Hakam ibn al-A’raj, he said: He informed me—or he said: He told me—the one who saw Abu Dharr, [that] his [arm] was blackened between his wrist and his elbow.