Musannaf Ibn Abi Shayba 16:629

Abu Bakr narrated to us, he said: Yahya bin Sa'id narrated to us, from Ibn Abi 'Aruba, from Ash'ath, from his father, that Abu Musa came out from the privy (al-khala') while he was wearing a skullcap (qalansuwa), and he wiped over it.