Musannaf Ibn Abi Shayba 3:4098

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Yunus bin Ubayd informed us, from Ibn Sirin, from Anas bin Malik, that Umar bin al-Khattab saw a man praying while he was wearing a cap whose lining was made of fox skins. He said: So he threw it off his head and said: "What makes you know? Perhaps it is not ritually pure (dhaki)."