From the Enchiridion — Epictetus

Some things are within our power, and some are not. Within our power are opinion, desire, aversion — in a word, whatever is our own doing. Not within our power are the body, property, reputation, office — whatever is not our own doing. The things within our power are by nature free, unhindered, and unrestrained; but those not in our power are weak, dependent, and belong to others. Remember, then, that if you take what is by nature another's to be your own, you will be hindered, you will grieve, and you will blame both gods and men. But if you take only what is truly yours to be your own, no one will ever compel you, no one will hinder you; you will blame no one, accuse no one, and not a single thing will happen against your will.