How can I get these kids to stop tattling on each other???

I love babysitting these kids, but they tattle on each other almost all the time! All I ever hear is, "She pushed me!" or, "She's being mean!" When this happens I say, "Hey! I do not listen to tattling. I will only listen to something that you did." But that doesn't work. What can I do to stop all of this unessessary tattling?