How do I limit my chracters rotation on the X axis?

I have set up a somewhat working Fps controller. Now I just need to limit the rotation on the X axis. I tried using an if statement but I can’t get it to work. How do I limit rotation on the X axis?

   Vector3 mouseInputY = new Vector3(Input.GetAxisRaw("Mouse Y"), 0.0f,0.0f);


        if (transform.rotation.x > 90f | transform.rotation.x < -90f)
        {
            null;
        }
        else
        {
            transform.Rotate(-mouseInputY * turnSpeed * Time.deltaTime);
        }

Don’t confuse | with ||, the latter is what you should be using.
Single dash is the binary operator, for doing all weird things with bytes

instead of null, use return or return null if applicable