If any of you leads the people in prayer, let him shorten it, because among them are the weak, the sick, and the busy. If any of you prays alone, then let him prolong (his prayer) as much as he wishes
Authentic hadithعن أبي هريرة رضي الله عنه مرفوعًا: «إذا صلى أحدكم للناس فَلْيُخَفِّفْ فإن فيهم الضعيف والسَّقِيمَ وذَا الحاجة، وإذا صلى أحدكم لنفسه فَلْيُطَوِّلْ ما شاء».
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If any of you leads the people in prayer, let him shorten it, because among them are the weak, the sick, and the busy (who have needs to fulfill). If any of you prays alone, then let him prolong (his prayer) as much as he wishes."
Explanation
The Prophet (may Allah's peace and blessings be upon him) commanded the Muslims to shorten the prayer according to what is in the Sunnah. He justified that by saying that the weak people, as well as the sick and busy people, pray behind them. However, when they pray individually, they are free to either prolong or shorten the prayer as they wish.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).