Sahih Muslim 467 b
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.
حَدَّثَنَا ابْنُ رَافِعٍ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، حَدَّثَنَا مَعْمَرٌ، عَنْ هَمَّامِ بْنِ مُنَبِّهٍ، قَالَ هَذَا مَا حَدَّثَنَا أَبُو هُرَيْرَةَ، عَنْ مُحَمَّدٍ، رَسُولِ اللَّهِ صلى الله عليه وسلم فَذَكَرَ أَحَادِيثَ مِنْهَا وَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ إِذَا مَا قَامَ أَحَدُكُمْ لِلنَّاسِ فَلْيُخَفِّفِ الصَّلاَةَ فَإِنَّ فِيهِمُ الْكَبِيرَ وَفِيهِمُ الضَّعِيفَ وَإِذَا قَامَ وَحْدَهُ فَلْيُطِلْ صَلاَتَهُ مَا شَاءَ ‏"‏ ‏.‏