The API does not make the raw source of a section available. The source is only stored at the block level.
What I've done in the past to extract the source of the section is to leverage the sourcemap. By enabling the sourcemap option, you get the file and line number for each section. Then, you take that information and go back to the original source of the document and use it to cut out the source for the second.
Here's some really rough code to show you what I'm talking about: