Musnad Ahmad 30:3176

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "When one of you leads the people in prayer, let him lighten it, for among them are the weak, the sick, and the elderly. And when one of you prays by himself, let him lengthen it as he wishes."