מדרש אויף שמות 5:8

“The king of Egypt said to them: Why, Moses and Aaron, do you disturb the people from their work? Go to your burdens” (Exodus 5:4).
“The king of Egypt said to them: Why, Moses and Aaron” – what is why [lama]?²⁹The question is difficult. Moses and Aaron had already explained why they were disturbing the people from their work. The midrash interprets the word lama as lema. Not “why” but rather, “what” are you and what are your words worth? He said to them: ‘What are you, and what are these matters?’
“Go to your burdens” – Rabbi Yehoshua ben Levi said: The tribe of Levi was unencumbered with the backbreaking labor. Pharaoh said to them: ‘Because you are unencumbered you say: “Let us go and sacrifice to our God” (Exodus 5:8); “go to your burdens.”’

Rabbi 'Aḳiba said: The taskmasters of Pharaoh were beating the Israelites in order that they should make the tale of bricks, and it is said, "And the tale of the bricks, which they did make heretofore, ye shall lay upon them" (Ex. 5:8). The Israelites were gathering the straw of the wilderness, and they were carrying it on their asses and (also on) their wives, and their sons. The straw of the wilderness pierced their heels, and the blood was mingled with the mortar. Rachel, the granddaughter || of Shuthelach, was near childbirth, and with her husband she was treading the mortar, and the child was born (there) and became entangled in the brick mould. Her cry ascended before the Throne of Glory. The angel Michael descended and took the brick mould with its clay, and brought it up before the Throne of Glory. That night the Holy One, blessed be He, descended, and smote the firstborn of the Egyptians, as it is said, "And it came to pass at midnight that the Lord smote all the firstborn in the land of Egypt" (Ex. 12:29).