Variable: adaptive-fill-regexp

Regexp to match text at start of line that constitutes indentation.
If Adaptive Fill mode is enabled, a prefix matching this pattern
on the first and second lines of a paragraph is used as the
standard indentation for the whole paragraph.

If the paragraph has just one line, the indentation is taken from that
line, but in that case `adaptive-fill-first-line-regexp' also plays a role.