Even though the proverb was not posted here, this question is still perfectly answerable.
Answer:
The rhetorical device Twain uses by referencing a well-known proverb is allusion.
Explanation:
Allusion is a figure of speech in which a reference is made to something or someone that has significance. The author does not explain much, since he/she assumes the audience knows who or what he/she refers to.
For instance, if someone says a woman is as beautiful as Helen of Troy, we would understand he/she means that woman is extremely beautiful. Helen of Troy is a famous character from the Iliad, by Homer, and she was the most beautiful woman in the world. The person making the allusion will not explain this fact, since it is well know.
Therefore, if Twain is referring to a well-known proverb, he is making an allusion to it.
