But a man’s best friend is the one who not only wishes him well but wishes it for his own sake (even though nobody will ever know it).

[φίλος δὲ μάλιστα ὁ βουλόμενος ᾧ βούλεται τἀγαθὰ ἐκείνου ἕνεκα, καὶ εἰ μηδεὶς εἴσεται]

Aristotle (384-322 BC) Greek philosopher

Nicomachean Ethics [Ἠθικὰ Νικομάχεια], Book 9, ch. 8 (9.8, 1168b.1) (c. 325 BC) [tr. Thomson/Tredennick (1976)]
    (Source)

(Source (Greek)). Alternate translations:

He is most a friend who wishes good to him to whom he wishes it for that man’s sake even though no one knows.
[tr. Chase (1847)]

A man's best friend is he who wishes him well for his own sake, without caring whether others are aware of his affection.
[tr. Williams (1869)]

The best friend is he who, when he wishes the good of another, wishes it for the other's sake, and wishes it even if nobody will know his wish.
[tr. Welldon (1892)]

He is most truly a friend who, in wishing well to another, wishes well to him for his (the other’s) sake, and even though no one should ever know.
[tr. Peters (1893)]

Man's best friend is one who wishes well to the object of his wish for his sake, even if no one is to know of it.
[tr. Ross (1908)]

The best friend is he that, when he wishes a person's good, wishes it for that person's own sake, even though nobody will ever know of it.
[tr. Rackham (1934)]

The one who is most a friend is the one who wishes good things to the person he wishes them to for that person's sake, even if no one will know.
[tr. Reeve (1948)]

A best friend is one who wishes the good of another for that other's sake, even if no one is to know this.
[tr. Apostle (1975)]

My best friend is the man who in wishing me well wishes it for my sake.