Short story

So there's this story, might be from Asimov, might not. I can't really remember. It's about an alien race from far, far off who find Earth and are like "these humans are pretty cool, but are probably dangerous" so they send a huge missile to destroy earth, but instantly regret it, and has the ending that's to the effect of the humans who survived Earth being destroyed saying, "We know you're out there, and we're coming for you."

Anyone know what I'm talking about? Name/link to the story please?