How do I list the files having particular strings from group of directories in bash?

How do I list the files having particular strings from group of
directories in bash?

I want to list files having strings like "hello", "how" and "todo" from a
directory (which is having multiple directories). Also I want to list only
c(.c) and cpp (.cpp) files only. I have tried with grep -R but not
satisfied. Please suggest me an alternate way. Thanks in advance.