How to limit mouse rotation at Z ?

Question

my code here but it don't help though :(

public class Pick : MonoBehaviour { public float RotationSpeed = 5;

 void Update ()
 {

 Vector3 mousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition)
 transform.rotation = Quaternion.LookRotation(Vector3.forward, mousePos - transform.position);
 }
}