THE ENCHIRIDION Ch. 14 (source: Project Gutenberg)
If you wish your children and your wife and your friends to live forever, you are foolish, for you wish things to be in your power which are not so, and what belongs to others to be your own. So likewise, if you wish your servant to be without fault, you are foolish, for you wish vice not to be vice but ]something else. But if you wish not to be disappointed in your desires, that is in your own power. Exercise, therefore, what is in your power. A man’s master is he who is able to confer or remove whatever that man seeks or shuns. Whoever then would be free, let him wish nothing, let him decline nothing, which depends on others; else he must necessarily be a slave.