(9) باب الإِشَارَةِ فِي الصَّلاَةِ
Narrated `Aisha the wife of the Prophet:
Allah's Messenger (ﷺ) during his illness prayed in his house sitting, whereas some people followed him standing, but the Prophet (ﷺ) beckoned them to sit down. On completion of the prayer he said, "The Imam is to be followed. So, bow when he bows, and raise your head when he raises his head." (See Hadith No. 657 Vol 1 for taking the verdict).
Reference: Sahih al-Bukhari 1236
In-book reference: Book 22, Hadith 14
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 22, Hadith 328