Musnad Abu Ya'la al-Mawsili 110:164

Abu Bakr ibn Abi Shaybah narrated to us, Abd al-Rahman ibn Muhammad al-Muharibi narrated to us, from al-Hajjaj, from al-Hakam, from Miqsam, from Ibn Abbas: That the Prophet, may Allah bless him and grant him peace, used to deliver the sermon on Friday while standing, then he would sit, then he would stand and deliver the sermon.