Sahih Muslim 1047
صحيح مسلم#1047SahihHammam 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 4:209