Enchiridion 6

Be not elated at any excellence which is not your own. If the horse in his elation were to say, "I am beautiful," it could be endured; but when you say in your elation, "I have a beautiful horse," rest assured that you are elated at something good which belongs to a horse. What, then, is your own? The use of external impressions. Therefore, when you are in harmony with nature in the use of external impressions, then be elated; for then it will be some good of your own at which you will be elated.

Epictetus, Enchiridion 6, translated by W. A. Oldfather

Be not elated at any advantage (excellence), which belongs to another. If a horse when he is elated should say, I am beautiful, one might endure it. But when you are elated, and say, I have a beautiful horse, you must know that you are elated at having a good horse. What then is your own? The use of appearances. Consequently when in the use of appearances you are conformable to nature, then you will be elated, for then you will be elated at something good which is your own.

Epictetus, Enchiridion 6, translated by George Long

Be not elated at an excellence which is not your own. If the horse in his pride were to say, 'I am handsome', we could bear with it. But when you say with pride, 'I have a handsome horse', know that the good horse is the ground of your pride. You ask then what you can call your own. The answer is—the way you deal with your impressions. Therefore when you deal with your impressions in accord with nature, then you may be proud indeed, for your pride will be in a good which is your own.

Epictetus, Enchiridion 6, translated by P. E. Matheson