Maysarah bin Habib (may Allah have mercy on him) narrates from Musa bin Talhah (may Allah have mercy on him), who said: I saw Sayyiduna Uthman (may Allah be pleased with him) delivering the sermon standing on the pulpit, and I saw Sayyiduna Mu'awiyah (may Allah be pleased with him) delivering the sermon sitting. He said: "Listen! I am not ignorant of the Sunnah, but I have grown old, my bones have become frail, and your needs have increased. So I wished to fulfil some of your needs while sitting, and then stand and take my share of the Sunnah."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16787