How to prove the first eigenfunction of Robin problem is strictly positive?

Question

I'm considering the following Robin problem. $$ \begin{cases}\Delta u-Au=-\lambda_{1}u & \text { on } \Omega \\ \frac{\partial u}{\partial n}+Bu=0 & \text { on } \partial \Omega.\end{cases} $$

Here $A,B$ are constans,$\lambda_{1}$ is the first eigenvalue.$\Omega$ is a domain with smooth boundary.

I wonder why $u$ is strictly positive on $\Omega$.I know we only need to show it doesn't change sign.

In fact I know $u$ doesn't change sign when it's the first eigenfunction of laplacian.But I don't know how to prove the same conclusion in this kind of operator and boundary condition.

Thanks in advance.