Sahih Muslim Book 4, Hadith Number 849.

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 narrated by Hamza b. Mughira by another chain of transmitters (but with the addition of these words): I made up my mind to hold Abd al-Rahman b. ‘Auf back, but the Messenger of Allah (may peace be upon him) said: Leave him.”