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.  

عَنْ مُحَمَّدٍ رَسُولِ اللَّهِ ﷺ فَذَكَرَ أَحَادِيثَ مِنْهَا وَقَالَ رَسُولُ اللَّهِ ﷺ إِذَا مَا قَامَ أَحَدُكُمْ لِلنَّاسِ فَلْيُخَفِّفِ الصَّلاَةَ فَإِنَّ فِيهِمُ الْكَبِيرَ وَفِيهِمُ الضَّعِيفَ وَإِذَا قَامَ وَحْدَهُ فَلْيُطِلْ صَلاَتَهُ مَا شَاءَ  

Yahya related to me from Malik from Abu'z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah ﷺ said, "When you lead people in the prayer, make it short, because there are among them some people who are weak, ill and old. But when you pray on your own, make it as long as you wish."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا صَلَّى أَحَدُكُمْ بِالنَّاسِ فَلْيُخَفِّفْ فَإِنَّ فِيهِمُ الضَّعِيفَ وَالسَّقِيمَ وَالْكَبِيرَ وَإِذَا صَلَّى أَحَدُكُمْ لِنَفْسِهِ فَلْيُطَوِّلْ مَا شَاءَ  

[Machine] And the Messenger of Allah ﷺ said: "When any of you leads the prayer for the people, let him shorten the prayer, for among them are the old, the weak, and the sick. But when he prays alone, let him prolong his prayer as much as he wishes."  

وَقَالَ رَسُولُ اللهِ ﷺ إِذَا مَا قَامَ أَحَدُكُمْ لِلنَّاسِفَلْيُخَفِّفِ الصَّلَاةَ فَإِنَّ فِيهِمُ الْكَبِيرَ وَفِيهِمُ الضَّعِيفَ وَفِيهِمُ السَّقِيمَ وَإِذَا قَامَ وَحْدَهُ فَلْيُطِلْ صَلَاتَهُ مَا شَاءَ