It is narrated from Ata (may Allah have mercy on him) that Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: "When you are the imam, keep the prayer light, for among the people are the elderly, the weak and those with needs; and when you are praying alone, lengthen it as much as you can. Recite in every prayer." What the Messenger of Allah (peace and blessings be upon him) let us hear, we have let you hear, and what he kept hidden from us, we have also kept hidden from you. A man asked him: If I do not recite anything more than Surah al-Fatihah, is that valid? He replied: "If you recite more than it, that is better, and if you suffice with it, your prayer will still be valid."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2474