Musannaf Ibn Abi Shayba 24:158

Waki' narrated to us, he said: Al-A'mash narrated to us, from Abdullah ibn Murrah al-Sulami, he said: Al-Ash'ath ibn Qays came and sat next to Ka'b ibn Ujrah in the mosque, and he placed one of his legs upon the other. Ka'b said to him: "Put it down, for it is not appropriate for a human being."