The preface to Elements of Statistical Learning opens with the popular quote
In God we trust, all others bring data. — William Edwards Deming
The footnote to the quote is better than the quote:
On the Web, this quote has been widely attributed to both Deming and Robert W. Hayden; however Professor Hayden told us that he can claim no credit for this quote,and ironically we could find no “data” confirming that Deming actually said this.
The fact that so many people attributed the quote to Deming is evidence that Deming in fact said it. It’s not conclusive: popular attributions can certainly be wrong. But it is evidence.
Another piece of evidence for the authenticity of the quote is the slightly awkward phrasing “all others bring data.” The quote is often stated in the form “all others must bring data.” The latter is better, which lends credibility to the former: a plausible explanation for why the more awkward version survives would be that it is what someone, maybe Deming, actually said.
The inconclusive evidence in support of Deming being the source of the quote is actually representative of the kind of data people are likely to bring someone like Deming.