If you desire power, desire nothing
but contentment, which is its own kingdom.

Sa'adi (1184-1283/1291?) Persian poet [a.k.a. Sa'di, Moslih Eddin Sa'adi, Mushrif-ud-Din Abdullah, Muslih-ud-Din Mushrif ibn Abdullah, Mosleh al-Din Saadi Shirazi, Shaikh Mosslehedin Saadi Shirazi]

Gulistān [Rose Garden, گُلِستان], ch. 2 “On the Morals of Dervishes,” Story 28 (1258) [tr. Rehatsek/Newman (2004)]
    (Source)

Alternate translations:

If you want riches, seek only for contentment which is inestimable wealth.
[tr. Gladwin (1806)]

Wouldest thou be rich, seek, but content to gain;
For this a treasure is that ne'er will harm.
[tr. Eastwick (1852)]

If thou wishest for power, covet nothing
Except contentment which is sufficient happiness.
[tr. Burton (1888)]

If thou covetest riches, ask not but for contentment, which is an immense treasure.
[tr. Ross (1900)]

Seek not, if thou desire riches,
Aught but contentment, for it is an agreeable treasure.
[tr. Platts (1904)]