From Marcus Aurelius' Meditations (Book 4, Meditation 3):
People seek special retreats for themselves such as country houses, seashores, mountains. And you too often long for these things. But this is really simple-minded, for you have the power to retire into yourself any time you choose and rest. There is no place freer from stress than deep within your own soul – especially if you have furnished it with thoughts that offer perfect tranquility as soon as you look at them again. By tranquility I mean good spiritual order, no more, no less. Allow yourself this retreat often. Go there to renew yourself.