The question was once put to him, how we ought to behave to our friends; and the answer he gave was, “As we should wish our friends to behave to us.”

[ἐρωτηθεὶς πῶς ἂν τοῖς φίλοις προσφεροίμεθα, ἔφη, “ὡς ἂν εὐξαίμεθα αὐτοὺς ἡμῖν προσφέρεσθαι.”]

Aristotle (384-322 BC) Greek philosopher

Attributed in Diogenes Laërtius, Lives and Opinions of Eminent Philosophers [Vitae Philosophorum], Book 5, sec. 11 [tr. Yonge (1853)]
    (Source)

(Source (Greek)). Alternate translations:

To the question how we should behave to friends, he answered, "As we should wish them to behave to us."
[tr. Hicks (1925), sec. 21]

When asked how we should act towards friends, he said “as we would pray they act towards us!”
[tr. @sentantiq (2016)]

When asked how we should behave to friends, he said, "As we would wish them to behave to us."
[tr. Mensch (2018)]