Horace, Art of Poetry 136-142
nec sic incipies, ut scriptor cyclicus olim:
‘fortunam Priami cantabo et nobile bellum.’
quid dignum tanto feret hic promissor hiatu?
parturient montes, nascetur ridiculus mus.
quanto rectius hic, qui nil molitur inepte:
‘dic mihi, Musa, virum, captae post tempora Troiae
qui mores hominum multorum vidit et urbes.’
And you shouldn’t start, as a writer of cyclic epic once did, ‘Of Priam’s fate I sing and a famous war.’ What could he bring forth great enough to match such an opening promise? The mountains will go into labour and bring forth… a ridiculous mouse! How much better this man is, who doesn’t exert himself inappropriately: ‘Tell me, o Muse, of the man who, after the time of Troy’s capture, saw many men’s customs and cities.’
The final lines are, of course, a translation of the opening of the Odyssey.