Sahih Muslim Book 4, Hadith Number 847.

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.

Sahl b. Sa’d al-Sa’idi reported: The Apostle of Allah (may peace be upon him) went to Bani Amr b. ‘Auf in order to bring about reconciliation amongst them. The rest of the Hadith is the same but with (the addition of these words): “The Messenger of Allah (may peace be upon him) came and made his way through the rows till he came to the first row and Abu Bakr retraced his steps.”