We can have no better clue to a man’s character than the company he keeps.
Niccolò Machiavelli (1469-1527) Italian politician, philosopher, political scientist
The Discourses on Livy, Book 3, ch. 34 (1517) [tr. Thomson (1883)]
(Source)
Alt. trans.: "There is no better indication of a man's character than the company which he keeps."