Stultum facit fortuna, quem vult perdere.
Translation: Whom Fortune wishes to destroy she first makes mad.
Maxim 911. One of the most famous renditions of the ancient Greek proverb (which is anonymous and dates to the 5th century BCE or earlier).
The provenance of the proverb and its English versions is at Wikipedia's Euripides page, under the heading "Misattributed".