Musnad Abu Ya'la al-Mawsili 105:20

Ibn Numayr narrated to us, my father narrated to us, al-‘Ala’ ibn Salih narrated to us, from ‘Adi ibn Thabit, Abu Rashid narrated to us, he said: ‘Ammar ibn Yasir delivered a sermon to us, and he was brief in the sermon. He said: "Indeed, the Messenger of Allah (may Allah bless him and grant him peace) forbade us from lengthening the sermon."