Rufus. From the remarks of Epictetus on friendship
1Of things that are, God has put some under our control, and others not under our control. Under our control He put the finest and most important matter, that, indeed, by virtue of which He Himself is happy, the power to make use of external impressions. For when this power has its perfect work, it is freedom, serenity, cheerfulness, steadfastness; it is also justice, and law, and self-control, and the sum and substance of virtue. But all other things He has not put under our control. Therefore we also ought to become of one mind with God, and, dividing matters in this way, lay hold in every way we can upon the things that are under our control, but what is not under our control we ought to leave to the Cosmos, and gladly resign to it whatever it needs, be that our children, our country, our body, or anything whatsoever.
Source: Stobaeus, II. 8, 30. Musonius, frag. 38
1 The natural way to take this and the next few titles is to assume that Epictetus had quoted with approval a fairly long passage from his revered teacher Musouius Rufus.