מדרש אויף בראשית 41:10

(Est. 2, 21) In those days, while Mordccai was sitting in the king's gate, Bigthan and Theresh became wroth. R. Acha b. Abba, in the name of R. Jochanan, said: "The Holy One, praised be He! causes masters to be wroth against their servants, in order to do the will of the just, and He also causes the wrath of slaves against their masters, in order to do the will of the just." The wrath of the masters against their servants, as it is written (Gen. 41, 10) Pharaoh was wroth with his servant; to do the will of the just, as it is written. And there was with us a Hebrew lad, etc. And he makes slaves wroth against their masters, as it is written (Ext. 2, 21) In those days Bigthan and Theresh became wroth. To do the will of the just. This refers to Mordecai, as it is written, And the thing became known to Mordecai. R. Jochanan said Bigthan and Theresh were Tarsians, and spoke their own language. They spoke between themselves: Since Esther has come into the court, our eyes have not seen any sleep; let us put poison into the king's drink, in order that he should die." And they knew not that Mordecai was a member of the Great Sanhedrin, every one of whom knew seventy languages. Then Bigthan said to Theresh: "But my hour of duty is not the same as yours." "I will watch for you, too," answered Theresh. Then we understand the passage, And the thing was inquired into, and found true, to mean that he had not been at his post.

“In those days, Mordekhai was sitting at the king's gate; two of the king's officials, Bigtan and Teresh, among the doorkeepers, became angry and sought to lay hands on King Aḥashverosh” (Esther 2:21).
“In those days, Mordekhai was sitting at the king's gate; two of the king's officials, Bigtan and Teresh […became angry]” – what was the anger there? He relieved two and set one in their place; he relieved two aristocrats who were the doorkeepers and set that barbarian [in their place].¹⁸They took Mordekhai to be a barbarian.
“Was sitting at the king's gate” – Rabbi Berekhya said in the name of Rabbi Levi: It is written: “Come behold the works of the Lord, who made desolations on the earth” (Psalms 46:9). He incited servants against their master, to bestow greatness upon the righteous; that is what is written: “Bigtan and Teresh became angry…” – to bestow greatness upon Mordekhai. He incited masters against their servants to bestow greatness upon Joseph; that is what is written: “Pharaoh became angry with his servants” (Genesis 41:10).

Rabbi Yuda bar Simon and Rabbi Ḥanin in the name of Rabbi Yoḥanan: It is written: “Come behold the works of the Lord…” (Psalms 46:9) – the Holy One blessed be He caused masters to become angry at their servants to grant prominence to the righteous: “Pharaoh became angry at his servants” (Genesis 41:10) – to grant prominence to Joseph. [And He caused] servants [to become angry] at their masters in order to grant prominence to the righteous: “Bigtan and Teresh became angry” (Esther 2:21) – to grant prominence to Mordekhai.
Rabbi Yudan was speaking about these actions of Bigtan and Teresh. Rav said: They placed a short dagger in their shoes. Rabbi Ḥanin said: They prepared an apparatus to strangle him. Shmuel said: They placed serpent’s venom in his cup. “The matter was investigated, and was discovered” (Esther 2:23). The Rabbis say: They placed a fly in his goblet;⁶This was a poisonous fly (Etz Yosef; cf. Rabbi David Luria). immediately, “the matter was investigated, and was discovered.”