Castigans alium, te castiga prius ipsum,
Ut, castigatus, sis castigare paratus.
Source: Florilegium Gottingense (ed. Voigt), 310. Meter: Dactylic Hexameter. Note the internal rhymes: alium-ipsum ... castigatus-paratus.
When correcting another person (castigans alium), first correct (castiga prius) yourself first (te ipsum) so that you are ready (ut sis paratus) to correct another (castigare), having been yourself corrected (castigatus).
The vocabulary is keyed to the DCC Latin Vocabulary list. There is only one word in this poem that is not on the DCC list:
castīgo, castīgāre: chastise, reprimand, correct
alius -a -um: other, another
ipse, ipsa, ipsum: him- her- itself
paro -āre: prepare, acquire; parātus -a -um, ready
prior -ius: earlier, preceding; prius or priusquam, before
tū tuī tibi tē: you (sing.)
ut, uti: as (+ indic.); so that, with the result that (+ subj.)