He has taken it from one of the pastoral poems of Virgil.
It's a poem deeply invested in the tradition of pastoral poetry, pastoral poetry from Theocritus and Virgil onward.
Virgil alludes to just this passage in Homer when he describes the entrance of a whole crowd of people in to the underworld as "the falling of leaves in the early frost of autumn."
Here is the story, in brief. A lonely pilgrim—it is Dante himself—strays into a dark wood, and is guided by the Roman poet Virgil down into Hell to contemplate the harrowing fate of the damned.
