Zeus, who guided mortals to be wise,
has established his fixed law —
wisdom comes through suffering.
Trouble, with its memories of pain,
drips in our hearts as we try to sleep,
so men against their will
learn to practice moderation.
Favours come to us from gods
seated on their solemn thrones —
such grace is harsh and violent.

τὸν φρονεῖν βροτοὺς ὁδώ-
σαντα, τὸν [πάθει μάθος]
θέντα κυρίως ἔχειν.
στάζει δ’ ἀνθ’ ὕπνου πρὸ καρδίας
μνησιπήμων πόνος· καὶ παρ’ ἄ-
κοντας ἦλθε σωφρονεῖν.
δαιμόνων δέ που χάρις βίαιος
σέλμα σεμνὸν ἡμένων.

Aeschylus (525-456 BC) Greek dramatist (Æschylus)
Agamemnon, ll. 175-183 [tr. Johnston (2007)]
    (Source)

Alt. trans.:

• "It is through suffering that learning comes." [In Arnold Toynbee, "Christianity and Civilization" (1947), Civilization on Trial (1948)]
• "God, whose law it is that he who learns must suffer. And even in our sleep pain that cannot forget, falls drop by drop upon the heart, and in our own despite, against our will, comes wisdom to us by the awful grace of God." [tr. Hamilton (1930)]
• "Guide of mortal man to wisdom, he who has ordained a law, knowledge won through suffering. Drop, drop -- in our sleep, upon the heart sorrow falls, memory’s pain, and to us, though against our very will, even in our own despite, comes wisdom by the awful grace of God." [tr. Hamilton (1937)]

The first Hamilton alternate was used, slightly modified, by Robert Kennedy in his speech on the assassination of Martin Luther King, Jr. (4 Apr 1968). Kennedy's family used it as an epitaph on his grave Arlington National Cemetery: "Even in our sleep, pain which cannot forget, falls drop by drop upon the heart, until in our own despair, against our will, comes wisdom, through the awful grace of God."

See here for more discussion.