Sahih Muslim Book 4, Hadith Number 946.

Chapter : Recitation in the night prayer.

Uthman b. Abu’l-‘As at-Thaqafi reported: The Apostle of Allah (may peace be upon him) said to him: Lead your people in prayer. I said: Messenger of Allah. I perceive something (disturbing) in my soul. He (the Holy Prophet) asked me to draw near him and making me sit down in front of him he placed his hand on my breast between my nipples, and then, telling me to turn round, he placed it on my back between my shoulders. He then said: Act as an Imam for your people. He who acts as Imam of the people, he must be brief, for among them are the aged, among them are the sick, among them are the weak, and among them are the people who have business to attend. But when any of you prays alone, he may pray as he likes.