In the question " What is this lognormal-like distribution? ", ck1987pd stated the formula for the probability distribution of hitting times for a Brownian Motion, starting at $0$ with barriers at $c$ and $−c$ and step size $l=1$
ck1987pd also stated the result in their answer to this question: " Two-sided hitting time of Brownian motion ".
I am looking for a reference that derives this probability distribution. Does anyone know of such a reference?