When asked to define a friend, he said, “One soul dwelling in two bodies.”

[ἐρωτηθεὶς τί ἐστι φίλος, ἔφη, “μία ψυχὴ δύο σώμασιν ἐνοικοῦσα.”]

Aristotle (384-322 BC) Greek philosopher

Attributed in Diogenes Laërtius, Lives and Opinions of Eminent Philosophers [Vitae Philosophorum], Book 5, sec. 11 [tr. Mensch (2018)]
    (Source)

(Source (Greek)). Alternate translations:

He was once asked what a friend is; and his answer was, “One soul abiding in two bodies.”
[tr. Yonge (1853)]

To the query, "What is a friend?" his reply was, "A single soul dwelling in two bodies."
[tr. Hicks (1925), sec. 20]

When he was asked what a friend is, he replied “one soul occupying two bodies.”
[tr. @sentantiq (2016)]