Musannaf Ibn Abi Shayba 3:4765

7142. Khalid ibn Makhlad narrated to us, from Sulayman ibn Bilal, from Ja'far ibn Muhammad, who said: I heard al-Qasim ibn Muhammad saying: Mu'awiyah said: The Messenger of Allah (peace be upon him) said: "If the leader prays sitting, then pray sitting." He said: The people were amazed at the truthfulness of Mu'awiyah.