Musannaf Ibn Abi Shayba 34:66

Ubayd Allah ibn Musa narrated to us, Shayban narrated to us, from al-A’mash, from ‘Amr ibn Murrah, from ‘Abd al-Rahman ibn Abi Layla, who said: I was sitting near Ka’b ibn ‘Ujrah on the day of Jumu’ah, and al-Dahhak ibn Qays delivered a sermon to us while sitting. He (Ka’b) said: "Do you not see? By Allah, I have never seen an Imam of a Muslim people deliver a sermon while sitting."