Parashas Vayeira · Passuk 21:9
And Sarah saw the son of Hagar the Egyptian, whom she had borne to Avraham, mocking.
Torah Temimah points to Tosefta / midrashic collection (as cited; parallels in various sources).
TT lemma: מצחק.
English: “Mocking.” It was taught: Rabbi Shimon ben Yochai says in the name of Rabbi Akiva: The “laughter” spoken of here is only idolatry… Rabbi Eliezer says: This is sexual immorality… Rabbi Yehoshua says: This is bloodshed… And I say: Heaven forbid that there should be in the house of that righteous man anyone who does such things; rather, this “laughter” is only about inheritance—Yishmael was mocking and saying, “I am the first-born and I take a double portion.”
Derash on the verse (baraita / midrash) — not a formal named middah of the 13. Competing interpretations of a single verb are weighed against the character of the household.
The “mocking” of Yishmael is explained by some as idolatry, immorality or bloodshed, but the preferred reading is a claim of first-born inheritance rights that threatened Yitzchak.