From Sahih Muslim (Hadith 2001):
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
Reference: Muslim 2001