Midrasz do Hioba 15:5

Pesachim (Fol. 3) R. Joshua b. Levi said: "A man should never bring forth from his mouth an unfit word, for the Scripture uses a circumlocution of eight [extra] letters in order to avoid an ugly word; for it is said (Gen. 4, 2) Of the clean beasts and of the beasts that are not clean." R. Papa said: "A circumlocution of nine letters is found in the Scripture; for it is said (Deu. 23, 11) If there be among thee, any man that is not clean." Rabina said: "Ten letters [is the circumlocution]; for in the same text there is also an extra letter, Vav." R. Acha b. Jacob said: "Sixteen extra letters are used [in order to avoid an unfit word], as it is said (I Sam. 20, 26) For he thought something hath befallen him, he is not clean, because he has not yet purified himself [in order to avoid the word impure]." At the academy of R. Ishmael, it was taught that a man should always speak in clean language, since the seat of a male zab the passage calls (Lev. 15, 9) saddle, and the seat of a female zab the passage calls (Ib.) sitting place. This principle is also supported by the following passage (Job 15, 5 ) So that thou choosest the language of the prudent. Also by another passage (Ib. 33, 3) And my lips utter knowledge clearly. Why does he cite all these passages [is not the first one sufficient]? You will perhaps say that this applies only to Biblical matters, but for Rabbinical matters any language might be used. Come, listen; it is said, So that thou choosest a language of the prudent. And if you will say that this applies only to Rabbinical matters, but for secular affairs, any language might be used. Come, Listen to this passage; And my lips utter knowledge clearly.