Midrash sa Genesis 45:12: Ein Yaakov (Glick Edition), Mekhilta DeRabbi Yishmael, Midrash Tanchuma & Shir HaShirim Rabbah

It is written (Gen. 45, 22) To each of them he gave changes of raiment; but to Benjamin he gave … five changes of raiment. Is it possible that that which gave trouble to Joseph's father (Ib. b) proved a stumbling block to Joseph? For Rabba b. Mehasia quoted R. Chama b. Guria, who said in the name of Rab: "On account of two Selaim worth of silk which Jacob bestowed on Joseph in preference to his other sons, the brothers became jealous of him, and brought about the entrance of our ancestors into Egypt." "This was a hint that from him would descend a man who would wear five royal garments," said R. Benjamin b. Jepheth, in the name of R. Elazar. Who is he? Mordecai, concerning whom it is written (Est. 8, 15) And Mordecai went out in a royal apparel of blue and white, and with a great crown of gold, and with a cloak of fine linen and purple. (Gen. 45, 14) And he fell upon his brother Benjamin's necks. How many necks had Benjamin? R. Benjamin b. Jepheth, in the name of R. Elazar, said: "He wept for the two Temples, that were destined to be in Benjamin's land and were doomed to destruction." And Benjamin wept upon his neck; i.e., he wept for the Tabernacle of Shilo, that was designated in Joseph's part of the land, and was destined to be destroyed. (Ib.) And behold, your own eyes see, and the eyes of my brother Benjamin. "This means," said R. Benjamin b. Jepheth in the name of R. Elazar, "that he (Joseph) said to them, 'Just as I have nothing in my heart against Benjamin, who took no part in my sale, so have I nothing against you, who sold me.'" (Ib.) It is my mouth the best things of Egypt? R. Benjamin b. Jepheth, in the name of R. Elazar, explained this: "What I speak with my mouth, I think in my heart." (Ib. 23) And to his father he sent after this manner with the best things of Egypt. What is the meaning of that speaketh unto you. R. Benjamin b. Jepheth, in the name of R. Elazar, said: "He sent him old wine, which strengthens the minds of old when they drink it."

And whence is it derived that they did not change their language? From (Ibid. 45:12) "… for the mouth that speaks to you" (speaks in the holy tongue), and (Exodus 5:3) "The G–d of the Hebrews revealed Himself to us, etc." and (Genesis 14:13) "And the survivor came and he told Avram the Hebrew, etc."

Then the Holy One, blessed be He, performed a miracle by restoring their souls to them. Joseph said: “Behold, your eyes see, and the eyes of my brother Benjamin, that it is my mouth that speaketh unto you (Gen. 45:12); for I am speaking to you in Hebrew.” They would not believe him until he bared his body and showed them the sign of the covenant (i.e., his circumcision). Why was all that necessary? When he (was sold into slavery and) left them, he did not possess any signs of maturity but now he stood before them like a king, with the mark of maturity upon him (his beard).