CHORUS: The fiercest anger of all, the most incurable
is that which rages in the place of dearest love.

[ΚΥΚΛΩΨ: Δεινή τις ὀργὴ καὶ δυσίατος πέλει,
ὅταν φίλοι φίλοισι συμβάλωσ᾽ ἔριν.]

Euripides (485?-406? BC) Greek tragic dramatist

Medea [Μήδεια], l. 520ff (431 BC) [tr. Vellacott (1963)]
    (Source)

Of the estrangement Jason and Medea. Some translations say this line is given by the chorus leader, not the chorus as a whole.

(Source (Greek)). Other translations:

How sharp their wrath, how hard to be appeas'd
When friends with friends begin the cruel strife.
[tr. Wodhull (1782)]

When friends with friends at variance kindle strife,
Fierce is their anger and immedicable.
[tr. Potter (1814)]

Terrible is that anger, and to assuage
Most difficult, when friends with friends join battle.
[tr. Webster (1868)]

There is a something terrible and past all cure, when quarrels arise 'twixt those who are near and dear.
[tr. Coleridge (1891)]

Dreadful is that anger and irremediable, when friends with friends kindle strife.
[tr. Buckley (1892)]

Awful is wrath, and past all balm of healing,
When they that once loved clash in feud of hate.
[tr. Way (Loeb) (1894)]

Dire and beyond all healing is the hate
When hearts that loved are turned to enmity.
[tr. Murray (1906)]

It is a strange form of anger, difficult to cure, when two friends turn upon each other in hatred.
[tr. Warner (1944)]

A terrible thing is temper and knows no cure
When dear ones wrangle and fall to fighting each other.
[tr. Podlecki (1989)]

Terrible and hard to heal is the wrath that comes when kin join in conflict with kin.
[tr. Kovacs (Loeb) (1994)]

Terrible is the anger and almost beyond cure, when strife severs those whom love once joined.
[tr. Davie (1996)]

Friend against friend! An anger most implacable!
[tr. Theodoridis (2004)]

Anger is frightening and hard to remedy
when loved ones join in strife with loved ones.
[tr. Luschnig (2007)]

When members of a family fight like this,
rage pushes them beyond all compromise.
[tr. Johnston (2008)]

It is a deinē anger and past all cure, whenever philoi fall to strife [eris] with philoi.
[tr. Coleridge / Ceragioli / Nagy / Hour25]