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, § 2 (1517) [tr. Thomson (1883)]
    (Source)

Alternate translations:

One can have no greater indication of a man than the company that he keeps.
[tr. Mansfield / Tarcov (1996)]

There can be no clearer indication about a man than the company he keeps.
[tr. Bondanella / Bondanella (1997)]