From Sahih Muslim (Hadith 1047):
Hammam b. Munabbih reported:This is what Abu Huraira transmitted to us from Muhammad the Messenger of Allah (ﷺ), and he narrated (some) ahadith out of (these narrations and one of them is this): The Messenger of Allah (ﷺ) 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
Reference: Muslim 1047