Musannaf Ibn Abi Shayba 3:4763

Abu Usama narrated to us, from Isma'il, from Qays ibn Fahd, who said: Our Imam became ill, so he prayed sitting, and we prayed by his prayer. Abu Hurayrah said: The Imam is a leader. If he prays standing, then pray standing, and if he prays sitting, then pray sitting.