hippie-expand-only-buffers

hippie-expand-only-buffers is a variable defined in `hippie-exp.el'.
Its value is

nil

Documentation:
A list specifying the only buffers to search (in addition to current).
Can contain both regexps matching buffer names (as strings) and major modes
(as atoms). If non-nil, this variable overrides the variable
`hippie-expand-ignore-buffers'.

You can customize this variable.