Quintilian, Training of the Orator 4.2.91
Quintilian advises the young lawyer not to get caught out when being economical with the truth – and cites a proverbial expression to support his point.
utrubique autem orator meminisse debebit actione tota quid finxerit, quoniam solent excidere quae falsa sunt: verumque est illud quod vulgo dicitur, mendacem memorem esse oportere.
But in either case the orator will have to remember, throughout the whole case, what he has made up, since falsehoods are liable to slip one’s mind. That common expression is true: ‘A liar needs to have a good memory.’