Read any of their books especially where they criticize the need for God. Darwin stated that his theory is purely materialistic. No need for God. In the case of Dawkins watch any of his debates with Christian scientists.
Stating that there is no need for God is not the same as saying God does not exists as a fact.
Theists claim that only a God could have produced all of these species, "it takes a more complex thing to make complex things."
Evolution by Natural Selection is a mechanism that disproves the above statement, it is a system that allows simple life to become complex life without the need for any interference by a complex thing (entity/machine/etc).
That statement that Dawkins, Darwin and all scientist make is that Evolution by Natural Selection means that
God is not necessary. This is
not the same as stating
God does not exist, and science does not need to disprove God it is not its job, it has no burden of proof.
The issue is once you take away the "God is necessary" position there is little else to hang you hat on regarding the God hypothesis. Additionally we have found 1 system where simple things can become complicated without the need of a complicated thing, how many more exist in the universe.
Note: Not sure Darwin himself would have stated this largely due to the respect he had for his Wife, who was a devote Catholic. Darwin simply stated the facts as he found them, and wrote about what he had done in the way of proving his Hypothesis (it became a theory later).
Darwin after having his idea sat on it for an age and tried disproving it and developing it, his book has sections in it stating methods in which his hypothesis would be proven wrong, on order that other scientists could continuing his work.
He was largely disinterested in the debate the church was having over his work and seen the whole thing as a distraction, although I always feel from reading his own words that he know his father would have been proud of him furthering the sceptics position. His father being a sceptic before Darwin was even born.