The Oxford Classical Dictionary does not have an entry on sophrosyne, but names this virtue in connection with the Stoics because sophrosyne is a virtue incorporating self-control and moderation. For the Stoics, sophrosyne, courage, prudence, and justice were all cardinal virtues.
The Dictionary of the History of Ideas says sophrosyne comes from an adjective Homer uses saophrōn 'of sound mind' for which reason the line from Juvenal: mens sana in corpore sano 'healthy mind in a healthy body' is sometimes used to clarify what sophrosyne is all about. Theognis uses sophrosyne as the opposite of hubris, according to Ethics in Thucydides: the Ancient Simplicity, by Mary Frances Williams. Theognis' use is sometimes political, where sophrosyne is a characteristic of the conservative order, and, at other times, sophrosyne is linked with justice.
"sophrosyn (Greek, self-control, temperance, soundness of mind) One of the cardinal virtues, consisting in a harmonious state of rational control of one's desires. In Aristotle the temperate person is one who can abstain or indulge appetites to the right degree without a severe effort of will; the person who needs the effort of will is not temperate, and needs to be continent...."
"sophrosyn" The Oxford Dictionary of Philosophy. Simon Blackburn. Oxford University Press, 2008
Obviously, there is much more on sophrosyne, but the links and references above should get you started.