Cicero, On Duties 3.108
Here’s the translation of the Euripides excerpt from a few posts back.
non enim falsum iurare periurare est, sed quod ex animi tui sententia iuraris, sicut verbis concipitur more nostro, id non facere periurium est. scite enim Euripides: “iuravi lingua, mentem iniuratam gero”.
For swearing to something false is not perjury, but if you swear ‘from the inclination of your mind’ – as it is verbally expressed in our law – but then do not perform your oath, that is perjury. Remember Euripides writes, ‘I have sworn with my tongue, but I bear an unsworn mind.’