I had huge difficulty getting this to work for a shell variable containing a filename with whitespace. For some reason using:
file="foo bar/baz"
scp [email protected]:"'$file'"
as in @Adrian's answer seems to fail.
Turns out that what works best is using a parameter expansion to prepend backslashes to the whitespace as follows:
file="foo bar/baz"
file=${file// /\\ }
scp [email protected]:"$file"