No need to find. If you are just looking for a pattern within a specific directory, this should suffice:
grep -hn FOO /your/path/*.bar
Where -h is the parameter to hide the filename, as from man grep:
-h, --no-filename
Suppress the prefixing of file names on output. This is the default when there is only one file (or only standard input) to search.
Note that you were using
-H, --with-filename
Print the file name for each match. This is the default when there is more than one file to search.