Musannaf Ibn Abi Shayba 3:2805

Ghundar narrated to us, from Shu'bah, from Mansur, from 'Amr ibn Murrah, from Abu 'Ubaydah, from Ka'b ibn 'Ujrah, who said: He entered the mosque while 'Abd al-Rahman ibn Umm al-Hakam was delivering the sermon while sitting. He said, "Look at this wicked man; he delivers the sermon while sitting, while Allah the Exalted says: 'And when they saw a transaction or a diversion, they dispersed to it and left you standing.'"