There are many people who can exercise virtue in their own affairs, but are unable to do so in their relations with others. This is why the aphorism of Bias, “Office will reveal the man”, seems a good one, since an official is, by virtue of his position, engaged with other people and the community at large.

[πολλοὶ γὰρ ἐν μὲν τοῖς οἰκείοις τῇ ἀρετῇ δύνανται χρῆσθαι, ἐν δὲ τοῖς πρὸς ἕτερον ἀδυνατοῦσιν. καὶ διὰ τοῦτο εὖ δοκεῖ ἔχειν τὸ τοῦ Βίαντος, ὅτι ἀρχὴ ἄνδρα δείξει: πρὸς ἕτερον γὰρ καὶ ἐν κοινωνίᾳ ἤδη ὁ ἄρχων.]

Aristotle (384-322 BC) Greek philosopher

Nicomachean Ethics [Ἠθικὰ Νικομάχεια], Book 5, ch. 1 (5.1.15-16) / 1129b.33ff (c. 325 BC) [tr. Crisp (2000)]
    (Source)

(Source (Greek)). Alternate translations:

I mean, there are many who can practise virtue in the regulation of their own personal conduct who are wholly unable to do it in transactions with their neighbour. And for this reason that saying of Bias is thought to be a good one, “Rule will show what a man is;” for he who bears Rule is necessarily in contact with others, i.e., in a community.
[tr. Chase (1847), ch. 2]

For many there be who can make good use of their virtue in their own matters, but not towards their fellow-man. And, hence, Bias would seem to have said well, saying that, "It is authority that shows the man." For whosoever is in authority stands ipso facto in relation to his fellow-man, in that he is a fellow-member of the body politic.
[tr. Williams (1869)]

For there are many people who are capable of exhibiting virtue at home, but incapable of exhibiting it in relation to their neighbors. Accordingly there seems to be good sense in saying of Bias that "office will reveal a man," for one who is in office is at once brought into relation and association with others.
[tr. Welldon (1892)]

For there are many who can be virtuous enough at home, but fail in dealing with their neighbours. This is the reason why people commend the saying of Bias, “Office will show the man;” for he that is in office ipso facto stands in relation to others, and has dealings with them.
[tr. Peters (1893)]

For many men can exercise virtue in their own affairs, but not in their relations to their neighbour. This is why the saying of Bias is thought to be true, that "rule will show the man"; for a ruler is necessarily in relation to other men and a member of a society.
[tr. Ross (1908)]

For there are many who can practise virtue in their own private affairs but cannot do so in their relations with another. This is why we approve the saying of Bias, "Office will show a man"; for in office one is brought into relation with others and becomes a member of a community.
[tr. Rackham (1934)]

For many people are able to use their virtue in what properly belongs to themselves, but unable to do so in issues relating to another person. And this is why Bias' saying, "ruling office shows forth the man," seems good, since a ruler is automatically in relation to another person and in a community with him.
[tr. Reeve (1948)]

I say this because there are plenty of people who can behave uprightly in their own affairs, but are incapable of doing so in relation to somebody else. That is why Bias's saying "Office will reveal the man" is felt to be valid; because an official is eo ipso in relation to, and associated with, somebody else.
[tr. Thomson/Tredennick (1976)]

For many people are able to use virtue in dealing with the members of their household, but in their affairs together regarding another, they are unable to do so. And on this account, the saying of Bias seems good, that "office will show the man." For he who rules is already in relation to another and within the community.
[tr. Bartlett/Collins (2011)]