Shemot 

שמות

The children of Israel kept the faith of their ancestors even when adherence to that faith resulted in onerous enslavement. Even when Pharaoh decreed that every son born to the Israelites would be cast into the Nile, they continued to circumcise their sons. 

When the Israelites were in Egypt, they were upstanding…they gathered and sat as one group. They entered a covenant that they would perform acts of kindness with each other; that they would observe in their hearts the covenant God entered with Abraham, Isaac, and Jacob; and that they would not forsake the language of Jacob’s house and learn the Egyptian language, due to avoiding the ways of idol worship.  The Egyptians would say to them: Why do you worship your God? If you worship the gods of Egypt, Pharaoh will release you from labor. The Israelites responded and said to them: Did Abraham, Isaac, and Jacob forsake our God who is in heaven, such that it would also be justified for their descendants to forsake Him after them? The Egyptians said to them: No. The Israelites said to them: Just as they did not forsake Him, so we will not forsake Him. The Israelites would circumcise their sons in Egypt. The Egyptians said to them: What, will that cause Pharaoh to ease the hard labor of enslavement from you? The Israelites responded and said to them: Did Abraham, Isaac, and Jacob forget the covenant of our God who is in heaven, such that it would also be justified for their descendants to forget the covenant after them? The Egyptians said to them: No. The Israelites said to them: Just as they did not forget the covenant of our God who is in heaven, so their descendants after them will not forget.  <source>(<i>Eliyahu</i> <i>Rabba</i> [Ish Shalom] 21) </source>