Allow users access only to their own data in Firebase database?

I'm trying to have a data structure like this and to ensure that a user can only pull in their own data, since all the processing is done client side.

What database security rules would I have to use so that User1 can access their own posts, but cannot access User2's posts?

(I'm using Firebase web)

Sample database structure:

{
  "posts" : {
    "001" : {
      "text" : "note 1",
      "userID" : "User1"
    },
    "002" : {
      "text" : "note 2",
      "userID" : "User1"
    },
    "003" : {
      "text" : "note 3",
      "userID" : "User2"
    }
  }
}

Sample database query:

firebase.database().ref('/posts/').once('value').then(function(snapshot) {
  console.log(snapshot.val()); // Returns all 3 posts
});