Musnad Ahmad 30:3391

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you is an Imam, let him lighten [the prayer], for behind him stand the weak, the elderly, and those with needs. And when he prays by himself, let him lengthen [it] as much as he wishes."