transition_post_status

// Add the hook action
add_action('transition_post_status', 'send_new_post', 10, 3);

// Listen for publishing of a new post
function send_new_post($new_status, $old_status, $post) {
  if('publish' === $new_status && 'publish' !== $old_status && $post->post_type === 'post') {
    // Do something!
  }
}