It is narrated from Abu Huraira (may Allah be pleased with him) that the Prophet (peace be upon him) said: "When any of you leads the prayer, he should make it light, because there are sick and elderly people in the congregation. But when he is praying alone, he may make it as long as he wishes."
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 283
Hadith Takhrijاخرجہ البخاری: الاذان، باب اذا صلى لنفسه فليطول ماشاء: 703، ومسلم، الصلاة، باب امر الائمة بتخفیف الصلاة فی تمام: 467۔