Sahih Muslim Book 4, Hadith Number 848.

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.

Mughira b. Shu’ba reported that he participated In the expedition of Tabuk along with the Messenger of Allah (may peace be upon him). The Messenger of Allah (may peace be upon him) went out to answer the call of nature before the morning prayer and I carried along with him a jar (full of water). When the Messenger of Allah (may peace be upon him) came back to me (after relieving himself). I began to pour water upon his hands out of the jar and he washed his hands three times, then washed his face three times. He then tried to tuck up the sleeves of his cloak upon his forearms but since the sleeves were tight he inserted his hands in the cloak and then brought out his forearms up to the elbow below the cloak, and then wiped over his shoes and then moved on. Mughira said: I also moved along with him till he came to the people and (he found) that they had been saying their prayer under the Imamah of ‘Abd al-Rahman b. ‘Auf. The Messenger of Allah (may peace be upon him) could get one rak-ah out of two and said (this) last rak’ah along with the people. When Abd al-Rahman b. ‘Auf pronounced the salutation, the Messenger of Allah (may peace be upon him) got up to complete the prayer. This made the Muslims terrified and most of them began to recite the glory of the Lord. When the Apostle of Allah (may peace be upon him) finished his prayer, he turned towards them and then said: You did well, or said with a sense of joy: You did the right thing that you said prayer at the appointed hour.