Conquer, but never triumph.

[Siege, aber triumphire nicht.]

Marie von Ebner-Eschenbach (1830–1916) Austrian writer

Aphorisms [Aphorismen], No. 9 (1880) [tr. Wister (1883)]
    (Source)

(Source (German)). Another translation:

Be victorious but not triumphant.
[tr. Scrase/Mieder (1994)]