"For the strike against Egypt's firstborns. At the time when God sent the plague of the firstborns, Moses said to Pharaoh that at midnight, every firstborn would die. All the firstborns went to their fathers and said to them: 'All that Moses said has come upon us. If you do not ask that these Hebrews be removed from among us, we will die.'
Their fathers replied: 'Even if all the Egyptians die, they will not leave here.' What did the firstborns do? They went to Pharaoh and cried out to him, saying: 'Please, remove these people, or a calamity will come upon both us and you.' Pharaoh commanded his servants to seize and beat them.
What did the firstborns do? They immediately took up their swords and killed their fathers, as it is written, 'For the strike against Egypt's firstborns.' It does not say here, 'for the strike against the firstborns of Egypt,' but rather, 'for the strike against Egypt's firstborns.' Sixty myriads of firstborns killed their fathers.
Rabbi Avin said in the name of Rabbi Yudah ben Pazi: Pharaoh's daughter was also a firstborn, but she was saved by Moses' prayer, as it is written, 'Its value is greater than pearls; it cannot be compared with gold.' The word 'night' is written here. How do we know that it refers to the night that is sanctified to God?"