It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "When one of you leads people in prayer, he should keep the prayer brief, for among them are the old, the weak and the sick. If he prays alone, let him make the prayer as long as he wishes."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5277