Sahih Muslim Book 4, Hadith Number 943.

Chapter : Recitation in the night prayer.

Hammam b. Munabbih reported: This is what Abu Huraira transmitted to us from Muhammad the Messenger of Allah (may peace be upon him), and he narrated (some) ahadith out of (these narrations and one of them is this): The Messenger of Allah (may peace be upon him) said: When any one of you stands to lead people In prayer, he should shorten it, for amongst them are the aged, and amongst them are the weak, but when he prays by himself, he may prolong his prayer as he likes.