"No one is free who is not in control of himself" - Epictetus (?)

Οὐδεὶς ἐλεύθερος ἑαυτοῦ μὴ κρατῶν - Attributed to Epictetus (although this attribution is not certain), this one makes good Stoic sense - "No one is free who is not in control of himself."

In other words, a person who is ruled by his passions is a slave to them.