Spoken word equivalent for "paper does not refuse ink"

This phrase advises a healthy skepticism of the written word.

Is there a similar idiom that advises skepticism of the spoken word?

Solution 1:

Gregory Titelman, Random House Dictionary of America' Popular Proverbs and Sayings, second edition(2000) identifies a number of related expressions conveying this idea under the general title "Believe only half of what you see and nothing you hear":

Believe only half of what you see and nothing you hear. Question everything, especially rumors. The proverb has been traced back to Proverbs of Alfred (c. 1300). First attested in the United States in 1770. In 1845, it was used by the American poet Edgar Allan Poe (1809–49). It is found in varying forms: Believe nothing you hear and only half of what you see; Do not (don't) believe anything (everything) you hear nor (and) half of what you read; Never believe anything (everything) you hear; Believe only what they do and nothing of what they say; One must not believe all he hears; You can't believe everything you hear; Don't believe everything you read in the papers, etc. Usually they are followed by another old saying: "But I saw it. I saw it with my own two eyes."

For your purposes, "You can't believe everything you hear" may be the most relevant idiomatic form that offers a direct oral/aural counterpoint to the warning "Paper does not refuse ink" (which is essentially a fancier, more figurative way of saying, "Don't believe everything you read in the papers").

Wolfgang Mieder, A Dictionary of American Proverbs (1992) has some interesting alternative expressions along similar lines:

They say is a tough old liar. [also recorded as "The biggest liar in the world is they say.")

and  

Saying so don't make it so.

Solution 2:

You may also like talk is cheap:

[Merriam-Webster]
: used to mean that it is easy to say that one will do something

(where used here means is used to as opposed to implying an archaic meaning)

Solution 3:

There's an idiom that goes not worth the paper it's written on:

[Merriam-Webster]
: not of real value : not legally valid

(A variation is not worth the paper it's printed on.)

This has a similar connotation to the paper does not refuse ink saying from the question—which leads into my actual answer.

Lee Gesmer, a lawyer, wrote a blog post that added a clever twist to this sentiment in "An Oral Agreement is Only as Good as the Paper It's Written On." Meaning, in short, if it's not put into writing, then it's just words that that don't have any value.

For the purpose of this question, it could be modified slightly:

The spoken word is only as good as the paper it's written on.

The elegance of this is that not only does is it based on an existing idiom in relation to the written word, but it turns it around to refer to the skeptical nature of speech.

The two idioms actually go well together:

Paper does not refuse ink, and the spoken word is only as good as the paper it's written on.

In short, be skeptical of anything, written or spoken.

Solution 4:

Actions speak louder than words, which suggests that speech is less credible than actions, is widely used.

Another expression, with usage that extends beyond English, is A closed mouth catches no flies. It exists in many variants and in many languages, and sources attribute it to multiple people, but there seems to be no consensus about where it comes from.

One of this expression's variants, If you keep your mouth shut, the flies can't get in, is, I believe, a better answer to the question, because it implies that flies will just come in on their own. This suggests that someone talking too much would be caught unawares and would appear foolish (and probably be very uncomfortable) because of the fly influx. On the other hand, A closed mouth catches no flies suggests that someone might actually be trying to catch flies by speaking.

A Google search of A closed mouth catches no flies shows a long list of sites that attribute a range of origins to the expression and its variants:

• A Brazilian proverb (although this source seems rather questionable)
• An Arabic proverb (one French version: Dans une bouche close, n'entre point de mouche); Google Books shows an attribution to Cervantes.
• An Italian proverb
• A Spanish proverb

Additionally, Prosper Mérimée ends the second edition of his novella Carmen, which inspired Bizet's opéra-comique, with the expression En close bouche, n'entre point mouche (which corresponds roughly to the English If you keep your mouth shut, the flies can't get in).

The list goes on. Suffice it to say that this is a widely used idiom.