Here's my code:
registerAction() in controllers/SessionController.php
public function registerAction() {
        $form = new RegisterForm();
        if ($this->request->isPost()) {
            if ($this->security->checkToken()) {
                if ($form->isValid($_POST)) {
                        $user = new Users;
                        $user->username =   $this->request->getPost('username');
                        $user->email =      $this->request->getPost('email');
                        $user->password =   $this->security->hash($this->request->getPost('password'));
                        $user->created_at = time();
                        $user->create();
                }
                else
                {       
                    echo $form->getMessages()[0];
                }
            }
        }
        $this->view->form = $form;
}
models/User.php Model:
use Phalcon\Mvc\Model\Validator\Uniqueness;
class Users extends Phalcon\Mvc\Model
{
    public function validation()
    {
        $this->validate(new Uniqueness(array(
            'field' => 'email',
            "message" => "Value of field 'email' is already present in another record"
        )));
        $this->validate(new Uniqueness(array(
            'field' => 'username',
            "message" => "Value of field 'username' is already present in another record"
        )));
        if ($this->validationHasFailed() == true) {
            return false;
        }
    }
    public function initialize() {
        $this->skipAttributesOnCreate(array('status'));
    }
}
views/session/register.phtml
<?=$this->getContent();?>
<?php use Phalcon\Tag; ?>
<h1 class="main_heading">Create a new Account!</h1>
<div class="form">
    <form action="register" method="post">
        <p>
            <label for="username"><?=$form->getLabel('username');?></label>
            <?=$form->render('username');?>
        </p>
        <p>
            <label for="email"><?=$form->getLabel('email');?></label>
            <?=$form->render('email');?>
        </p>
        <p>
            <label for="email"><?=$form->getLabel('password');?></label>
            <?=$form->render('password');?>
        </p>
        <p>
            <input type="submit" value="Register" name="submit">
        </p>
        <input type="hidden" name="<?php echo $this->security->getTokenKey() ?>" value="<?php echo $this->security->getToken() ?>"/>
    </form>
</div>
forms/RegisterForm.php
use Phalcon\Forms\Form,
    Phalcon\Forms\Element\Text,
    Phalcon\Forms\Element\Password,
    Phalcon\Forms\Element\Submit,
    Phalcon\Forms\Element\Check,
    Phalcon\Forms\Element\Hidden,
    Phalcon\Validation\Validator\Regex,
    Phalcon\Validation\Validator\PresenceOf,
    Phalcon\Validation\Validator\StringLength,
    Phalcon\Validation\Validator\Email,
    Phalcon\Validation\Validator\Identical,
    Phalcon\Validation\Validator\Uniqueness;
class RegisterForm extends Form
{
    public function initialize()
    {
        /**
         *  Validate Username
         */
        $username = new Text('username', array(
            'placeholder' => 'Username'
        ));
        $username->setLabel('Username');
        $username->addValidators(array(
            new PresenceOf(array(
                'message' => 'Please enter a username.'
            )),
            new Regex(array(
                'pattern' => '/^[A-Za-z][A-Za-z0-9]*(?:_[A-Za-z0-9]+)*$/',
                'message' => 'Only letters, numbers and underscores allowed in the username'
            )),
            new StringLength(array(
              'max' => 16,
              'min' => 4,
              'messageMaximum' => 'Username too long. Max. 16 characters allowed.',
              'messageMinimum' => 'The username is too short. Please enter at least 4 characters.'
        ))
        ));
        $this->add($username);
        /**
         *  Validate E-Mail Address
         */
        $email = new Text('email', array(
            'placeholder' => 'Email'
        ));
        $email->setLabel('E-Mail');
        $email->addValidators(array(
            new PresenceOf(array(
                'message' => 'Please enter an email address.'
            )),
            new Email(array(
                'message' => 'Please enter a valid email address.'
            ))
        ));
        $this->add($email);
        /**
         *  Validate Password
         */
        $password = new Text('password', array(
            'placeholder' => 'Password'
        ));
        $password->setLabel('Password');
        $password->addValidators(array(
            new PresenceOf(array(
                'message' => 'You did not enter a password.'
            )),
            new StringLength(array(
              'max' => 40,
              'min' => 4,
              'messageMaximum' => 'The password must be max 40 characters.',
              'messageMinimum' => 'The password must be at least 4 characters.'
        ))
        ));
        $this->add($password);
    }
}