This is one, which I first saw on Twitter, I initially had almost no suspicions about. It seemed quite likely that the Buddha might have said something like this. But, as it turns out, he didn’t. Or at least it’s not been recorded.
Southern Folklore Quarterly, Volume 13, 1949, page 127, gives as the source of quote a book called “Seven Hundred Chinese Proverbs, by Henry H. Hart (Stanford University Press, c. 1937), and indeed many books refer to it as a Chinese saying. The Buddha was not Chinese, so this is, to say the least, unlikely to be a canonical Buddha quote. It’s more likely that someone, as so often seems to happen, decided to add the Buddha’s name to this quote at some point.
It’s a common enough image, though, and it’s possible that the Buddha said something like this. In Portuguese they have the saying, “A lingua não é de aço, mas corta,” and in Spanish they say, “La lengua del mal amigo más corta que cuchillo.”
Interestingly, I haven’t come across the the word “tongue” in Pali being used to represent speech. “Jivhā” literally means tongue, and also “taste,” as in “Jivhā indriya” (sense of taste). But it isn’t used metonymically to stand for “speech.” Or if it is, I haven’t encountered it. It’s funny how languages have different associations with something as simple as the tongue.
There is a similar image in the early scriptures, although it’s not said by the Buddha:
A person is born
with an axe in their mouth.
A fool cuts themselves with it
when they say bad words.
These words are supposed to have been said by Tudu, a “paccekabrahmā,” which is a god (brahma) who moves about alone (pacceka), without a retinue. He speaks these words to a disciple of the Buddha who has been badmouthing two of the Buddha’s chief disciples.
So, it’s a similar image to the tongue being a knife, and it’s in the scriptures, but it’s not said by the Buddha.