Here’s the relevant bit:
quid, si etiam iucunda memoria est praeteritorum malorum? ut proverbia non nulla veriora sint quam vestra dogmata. vulgo enim dicitur: 'Iucundi acti labores', nec male Euripides— concludam, si potero, Latine; Graecum enim hunc versum nostis omnes—: 'Suavis laborum est praeteritorum memoria.'
What if the memory of past evils be actually pleasant? Proving certain proverbs truer than the tenets of your school. There is a popular saying to the effect that ‘Toil is pleasant when ‘tis over’; and Euripides well writes (I will attempt a verse translation; the Greek line is known to you all): 'Sweet is the memory of sorrows past'.
Cicero is criticising the Epicureans’ insistence that memory of past psychic pleasures can counteract present physical pains. He notes that not everything that was painful in the past is painful to remember. He translates a line from Euripides (taken usually to be 133 Nauck 2nd edition). The Greek, found at Arist. Rhet. 1370b4 and at Plut. Quaest. Conv. 630E is: ἀλλ’ ἡδύ τοι σωθέντα μεμνῆσθαι πόνων.
Anyway, I was checking what some commentators say about this. Madvig says (I think; the pdf is a bit blurry):
nosti omnes Cicero dixit, oblitus videtur, quot sermoni interesse finxisset; nam ut intellegatur: vos omnes Epicurei, quemadmodum Gaius [sic?] volt, fieri non potest, quom, cur illi versum esse sententiae plane contrarium praeter ceteros memoria retinerent, nulla caussa fuerit.’ (This is the from the 1839 edition I found on the internets; I have checked later editions.)
It seems to me that Cicero could mean ‘all you Epicureans know the line’. If he does, then he poses a dilemma: either they remember the line but seem to have ignored the sentiment or they do not remember the line, in which case their powers of and control over their memory leaves something to be desired. It might also be interesting to wonder whether Cicero has quite remembered the line himself: the σωθέντα in Aristotle’s quotations does not appear in his version. Perhaps he’s being super clever and anyone who does remember the Euripides line in the original will also note that Cicero’s memory is less than accurate here.