Letters 85.39

So thoroughly, I mean, has he schooled himself that he makes manifest his virtue in prosperity as well as in adversity, and keeps his eyes on virtue itself, not on the objects with which virtue deals. Hence neither poverty, nor pain, nor anything else that deflects the inexperienced and drives them headlong, restrains him from his course.

Seneca, Letters to Lucilius 85.39, translated by Richard Mott Gummere