This relief from the Ara Pacis depicts the passage below.
"And that this nightly vision may not seem
Th' effect of fancy, or an idle dream,
A sow beneath an oak shall lie along,
All white herself, and white her thirty young.
When thirty rolling years have run their race,
Thy son Ascanius, on this empty space,
Shall build a royal town, of lasting fame,
Which from this omen shall receive the name."
Virgilius - The Aeneid VIII
translated by J. Dryden