Midrash for Pirkei Avot 2:10
הֵם אָמְרוּ שְׁלשָׁה שְׁלשָׁה דְבָרִים. רַבִּי אֱלִיעֶזֶר אוֹמֵר, יְהִי כְבוֹד חֲבֵרְךָ חָבִיב עָלֶיךָ כְּשֶׁלָּךְ, וְאַל תְּהִי נוֹחַ לִכְעֹס. וְשׁוּב יוֹם אֶחָד לִפְנֵי מִיתָתְךָ. וֶהֱוֵי מִתְחַמֵּם כְּנֶגֶד אוּרָן שֶׁל חֲכָמִים, וֶהֱוֵי זָהִיר בְּגַחַלְתָּן שֶׁלֹּא תִכָּוֶה, שֶׁנְּשִׁיכָתָן נְשִׁיכַת שׁוּעָל, וַעֲקִיצָתָן עֲקִיצַת עַקְרָב, וּלְחִישָׁתָן לְחִישַׁת שָׂרָף, וְכָל דִּבְרֵיהֶם כְּגַחֲלֵי אֵשׁ:
They said three things [in derech eretz, in mussar, and in middoth (for in the areas of forbidden-permitted, exempt-liable, they said many things. Or, in these three areas, there was always a "pearl" in their mouths.)] R. Eliezer says: Let the honor of your friend be as dear to you as yours and do not be easily moved to anger. [i.e., when is this possible? When you are not easily moved to anger. (This is one thing; for if you are easily moved to anger, it is impossible not to cheapen your friend's honor.)] And repent one day before your death. [This is the second thing. For since one does not know when he will die, he will repent today lest he die tomorrow.] And warm yourself at the fire of the sages. [This is the third thing.], but be heedful of their coal [not to be lightheaded before them] that you not be burned [i.e., so that you not be punished through them]. For their bite is the bite of a fox, [which is very difficult to heal. For its teeth are small, crooked, and slanted, and the surgeon cuts the flesh with a scalpel to widen the bite], and their sting is the sting of a scorpion, [which is worse than the bite of a snake], and their speech is the hiss of a fiery serpent. [which burns when it hisses. Alternately: The fiery serpent is not to be charmed as other snakes are, as it is written (Psalms 58:6): "He does not heed the voice of the charmers." So, a Torah scholar. If you antagonize him and come to appease him, he is not (easily) appeased], and all of their words are like coals of fire.