Midrash sa Exodo 10:8: Midrash Tanchuma, Mekhilta DeRabbi Yishmael & Sifra

Go serve the Lord, but who are they that shall go? (Exod. 10:8), he asked, and you replied: We shall go with our young and with our old. Then I say to you: Arise up, get you forth from among my people, both ye and the children of Israel (ibid. 12:31), only let your flocks and your herds remain (ibid. 10:24). You answered: Our cattle also shall go with us, there shall not a hoof be left behind (ibid., v. 26). Then take both your flocks and your herds, as ye have said, and be gone (ibid. 12:32). Would that you had already departed. Then you added: Thou must also give into our hands sacrifices and burnt-offerings (ibid., v. 28). Take whatever you wish and depart, and bless me also (ibid., v. 32); said Pharaoh.

"And he said: Arise and go out": I (Pharaoh) said (Ibid. 10:8) "Who and who are the goers?" and you said (Ibid. 9) "With our youth and with our elders will we go" —

36) (Vayikra 10:3): "And Moses said to Aaron: This is what the L–rd spoke, saying: With My near ones will I be sanctified, and before all the people will I be slaughtered. And Aaron kept his peace." This (in essence) was spoken to Moses on Sinai, and he did not know it (i.e., its intent) until what it spoke of (the deaths of Nadav and Avihu) materialized before him. And when it came to pass Moses said to Aaron: Aaron, my brother, your sons died only to sanctify the Name of the Holy One Blessed be He, as it is written (Shemoth 29:43): "And I shall be appointed there (in the tent of meeting), and it will be sanctified with My glory," (the death of Aaron's sons there, intensifying its awe in the eyes of the people). When Aaron saw how exalted were his sons in the eyes of the L–rd, he kept his peace and was rewarded for doing so, (by G d's addressing to him exclusively the following section [viz. Shemoth 10:8-11]). From here it was derived: All who accept (G d's decrees) upon themselves and remain silent — it is an auspicious sign for them. As expressed by David (Tehillim 37:7): "Be silent in the L–rd, and hope to Him." And, as expressed by Solomon (Koheleth 3:7): "a time to be silent and a time to speak." There are times for all (things). There are times when a man remains silent, and there are times when a man speaks.