Sahih Muslim Book 4, Hadith Number 846.

Chapter : The Imam is authorised to appoint one as his deputy when there is a valid reason for it (for example, illness or journey or any other), and if an Imam leads the prayer sitting as he cannot do so standing, his followers should say prayer standing provided they are able to do it and there is an abrogation of saying prayer sitting behind a sitting Imam.

This Hadith is transmitted by Sahl b. Sa’d in the same way as narrated by Malik, with the exception of these words: “Abu Bakr lifted his hands and praised Allah and retraced his (steps) till he stood in a row.”