Sahih Muslim 468 a
Uthman b. Abu'l-'As at-Thaqafi reported:
The Apostle of Allah (ﷺ) said to him: Lead your people in prayer. I said: Messenger of Allah. I perceive something (disturbing) in my soul. He (the Holy Prophet) asked me to draw near him and making me sit down in front of him he placed his hand on my breast between my nipples. and then, telling me to turn round, he placed it on my back between my shoulders. He then said: Act as an Imam for your people. He who acts as Imam of the people, he must be brief, for among them are the aged, among them are the sick, among them are the weak, and among them are the people who have business to attend. But when any of you prays alone, he may pray as he likes.
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عَمْرُو بْنُ عُثْمَانَ، حَدَّثَنَا مُوسَى بْنُ طَلْحَةَ، حَدَّثَنِي عُثْمَانُ بْنُ أَبِي الْعَاصِ الثَّقَفِيُّ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ لَهُ ‏"‏ أُمَّ قَوْمَكَ ‏"‏ ‏.‏ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ إِنِّي أَجِدُ فِي نَفْسِي شَيْئًا ‏.‏ قَالَ ‏"‏ ادْنُهْ ‏"‏ ‏.‏ فَجَلَّسَنِي بَيْنَ يَدَيْهِ ثُمَّ وَضَعَ كَفَّهُ فِي صَدْرِي بَيْنَ ثَدْيَىَّ ثُمَّ قَالَ ‏"‏ تَحَوَّلْ ‏"‏ ‏.‏ فَوَضَعَهَا فِي ظَهْرِي بَيْنَ كَتِفَىَّ ثُمَّ قَالَ ‏"‏ أُمَّ قَوْمَكَ فَمَنْ أَمَّ قَوْمًا فَلْيُخَفِّفْ فَإِنَّ فِيهِمُ الْكَبِيرَ وَإِنَّ فِيهِمُ الْمَرِيضَ وَإِنَّ فِيهِمُ الضَّعِيفَ وَإِنَّ فِيهِمْ ذَا الْحَاجَةِ وَإِذَا صَلَّى أَحَدُكُمْ وَحْدَهُ فَلْيُصَلِّ كَيْفَ شَاءَ ‏"‏ ‏.‏