I know that I suffer and this is no small pain:
Not to know, now that brings some pleasure to
The troubled — ignorance is an advantage amid grief.

[φρονῶ δ’ ὃ πάσχω, καὶ τόδ’ οὐ σμικρὸν κακόν·
τὸ μὴ εἰδέναι γὰρ ἡδονὴν ἔχει τινὰ
νοσοῦντα, κέρδος δ’ ἐν κακοῖς ἀγνωσία.]

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

Antiope [Αντιοπη], frag. 205 (Kannicht) (c. 410 BC) [tr. @sentantiq (2015)]
    (Source)

A source for the phrase, "Ignorance is bliss." (Source (Greek); see also TGF frag 204). Alternate translation:

I understand what I endure, and this
Is no small evil; for to the diseas'd
There is a kind of pleasure in not knowing
Their malady; such ignorance is gain
To those who labor under grievous woes.
[tr. Wodhall (1809); Barnes 23, Musgrave 24]

I understand what I suffer, and this is not a small evil:
for not to know that one is ailing has some pleasure,
in misery ignorance is an advantage.
[tr. Will (2015)]