Sahih Al Bukhari Hadith Number 1236

Narrated `Aisha the wife of the Prophet: Allah’s Messenger (peace be upon him) during his illness prayed in his house sitting, whereas some people followed him standing, but the Prophet (peace be upon him) 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).

22  Forgetfulness in Prayer
Hadith number 1236
Volume no 2
Chapter: 9 (Beckoning during the Salat [by a person in Salat])