“I believe in evidence. I believe in observation, measurement, and reasoning, confirmed by independent observers. I'll believe anything, no matter how wild and ridiculous, if there is evidence for it. The wilder and more ridiculous something is, however, the firmer and more solid the evidence will have to be.”

Source: What Got You Here Won't Get You There, 2008, p. 24 (in 2010 edition)

Episode 696: "Viewer Calls" http://www.youtube.com/watch?v=0OCYhDFc42I, Channel Austin (February 13, 2011)
The Atheist Experience
Context: Your position is... one where there is a god who has an important message for mankind, and somehow he only reveals it to certain individuals who then write this down and thousands of years after this initial revelation, we have to rely on copies of copies of translations of copies by anonymous authors with no originals, and the textual testimony to a miracle, for example the loaves and fishes; there’s no amount of reports - anecdotal testimonial reports - that could be sufficient to justify that this event actually happened as reported. No amount. And anything that would qualify as a god would clearly understand this, and if it wanted to convey this information to people in a way that was believable, would not be relying on text to do so, and this for me is the nail in the coffin for Christianity. The god that Christians believe in is amazingly stupid if it wants to actually achieve its goal of spreading this information to humanity by relying on text; by relying on languages that die out; by relying on anecdotal testimony. That's not a pathway to truth! And anything that would qualify for a god should know this, which means either that God doesn’t exist or it doesn't care enough about those people who understand the nature of evidence to actually present it. Now which of those possibilities do you think is accurate?"... "Why would you believe anything on faith? Faith isn't a pathway to truth. Every religion has some sort of faith, people take things on, you know, - if faith is your pathway, you can't distinguish between Christianity, Hinduism, Judaism, any of these others. How is it that you use reason as a path to truth in every endeavor of your life, and then when it comes to the ‘ultimate truth’ - the most important truth - you're saying that faith is required. And how does that reflect on a god (who supposedly exists and wants you to have this information); what kind of god requires faith instead of evidence?... I have reasonable expectations based on evidence. I have trust that has been earned. I will grant trust tentatively. I don't have faith. Faith is the excuse people give for believing something when they don't have evidence.

Sometimes attributed to Contact (1985), but the quote does not appear in that book.
It appears attributed to Sagan in Judson Poling's 2003 book "Do Science and the Bible Conflict?", but without source.
Misattributed
Source: Via Google Books https://books.google.com/books?id=Ondv5WzhYYYC&pg=PA21&dq=Sagan

Part 3, Section 16
A Treatise of Human Nature (1739-40), Book 1: Of the understanding

Source: A Dictionary of Thoughts, 1891, p. 497.

The Ethics of Belief (1877), The Duty of Inquiry
Context: To sum up: it is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence.
If a man, holding a belief which he was taught in childhood or persuaded of afterwards, keeps down and pushes away any doubts which arise about it in his mind, purposely avoids the reading of books and the company of men that call into question or discuss it, and regards as impious those questions which cannot easily be asked without disturbing it--the life of that man is one long sin against mankind.