Parashas Shemos
וַיֵּצֵא֙ בַּיּ֣וֹם הַשֵּׁנִ֔י וְהִנֵּ֛ה שְׁנֵֽי־אֲנָשִׁ֥ים עִבְרִ֖ים נִצִּ֑ים וַיֹּ֙אמֶר֙ לָֽרָשָׁ֔ע לָ֥מָּה תַכֶּ֖ה רֵעֶֽךָ׃
When he went out the next day, he found two Hebrews fighting; so he said to the offender, “Why do you strike your fellow?”
Torah Temimah points to R’ Yochanan in the name of R’ Shimon ben Yochai — Nedarim / TT (verified).
עברים נצים — א״ר יוחנן משום ר׳ שמעון בן יוחאי: כל מקום שנאמר נצים ונצבים אינם אלא דתן ואבירם.
English: “Hebrews quarreling (nitzim)” — R’ Yochanan said in the name of R’ Shimon ben Yochai: Wherever the language nitzim or nitzavim appears, it refers only to Dasan and Aviram.
Custom
Chazal identify the two quarreling Hebrews as Dasan and Aviram, the same men who would later oppose Moshe.