Googling cave idus Martias reveals sloganed t-shirts, high school Latin plays, and plenty of blog entries, but no citations from the classical authors.

Found this page by adding tu quoque (which Shagsberd turned into et tu, Brute) into the mix. According to Suetonius, the haruspex who does the warning had the name of Spurinna.

Quote:
Et immolantem haruspex Spurinna monuit, caveret periculum,
quod non ultra Martias Idus proferretur.

[...]

Dein pluribus hostiis caesis, cum litare non posset, introiit curiam spreta religione Spurinnamque irridens et ut falsum arguens, quod sine ulla sua noxa Idus Martiae adessent: quanquam is "venisse quidem eas" diceret, "sed non praeterisse."

The soothsayer Spurinna, observing certain ominous appearances in a sacrifice which he was offering, advised him to beware of some danger, which threatened to befall him before the ides of March were past.

[...]

Victim after victim was slain, without any favourable appearances in the entrails; but still, disregarding all omens, he entered the senate-house, laughing at Spurinna as a false prophet, because the ides of March were come without any mischief having befallen him. To which the soothsayer replied, "They are come, indeed, but not past."

[Ed. by Alexander Thomson.]

Ceci n'est pas un seing.