Extraordinary claims require extraordinary evidence Template:Quotation
Carl Sagan popularized this as "Extraordinary claims require extraordinary evidence".[1] However, this may have been based on a quote by Laplace which goes, "The weight of evidence for an extraordinary claim must be proportioned to its strangeness." This, in turn, may have been based on the statement "A wise man, therefore, proportions his belief to the evidence" by David Hume.[2]
References
Adapted from Wikipedia