Let $K_\alpha(t,x)$ be the (generalised or fractional) *Heat kernel* which corresponds to the fractional Heat equation (I'm not sure that's the right name) in $\mathbb R^n$
$$
u_t=(-\Delta)^\alpha u, \quad \alpha\in(0,1].
$$

A fact, which appears to be very well-known, states that: $$ K_\alpha(t,x)>0, \quad \text{for all $x\in\mathbb R^n,\, t>0$ and $\alpha\in(0,1]$ }. $$ It is so well-known that no paper where I have encountered this fact gives any reference.

I am looking for a reference where I could see the proof of this fact.