Imam Ja'far bin Muhammad (may Allah have mercy on him) narrates, through al-Qasim bin Muhammad (may Allah have mercy on him), from Sayyiduna Mu'awiyah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to the people: "If the imam prays sitting, then you too pray sitting." Al-Qasim (may Allah have mercy on him) said: "The people were amazed at the truthfulness of Sayyiduna Mu'awiyah (may Allah be pleased with him)."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16813