org-src-preserve-indentation

org-src-preserve-indentation is a variable defined in `org-src.el'.
Its value is

nil

Documentation:
If non-nil preserve leading whitespace characters on export.
If non-nil leading whitespace characters in source code blocks
are preserved on export, and when switching between the org
buffer and the language mode edit buffer. If this variable is nil
then, after editing with M-x org-edit-src-code, the
minimum (across-lines) number of leading whitespace characters
are removed from all lines, and the code block is uniformly
indented according to the value of `org-edit-src-content-indentation'.

You can customize this variable.