Chapter 8: The Book of Prayer - Friday
Ka' b b. 'Ujra reported that he entered the mosque and saw Abd al-Rahman b. Umm Hakam delivering the sermon in a sitting posture. Upon this he said: Look at this wretched person; he delivers the sermon while sitting, whereas Allah said: " And when they see merchandise or sport, they break away to it and leave thee standing."
Sahih Muslim 864