Only output the name of file containing a pattern (without the line output) using grep

I want to list the files that contain a pattern, without outputting the line(s) containing the pattern.  I assume that it's possible to do this with grep.  How should I use grep in that sense?  For example, the following command gives all the lines containing "stringpattern" (case insensitive) in all the .txt files.  I want to have only the name of the file (± the line number).

grep -ni stringpattern *.txt

Ideally, if the string/pattern is present more than once in one file, I would like to have multiple lines of output for that file.