Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1881
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."