You need t use -l flag with grep(lowercase L). From Unix Docs:
-l
(The letter ell.) Write only the names of files containing selected
lines to standard output. Pathnames are written once per file searched.
If the standard input is searched, a pathname of (standard input) will
be written, in the POSIX locale. In other locales, standard input may be
replaced by something more appropriate in those locales.