Rashi on Exodus 12:30/רש"י על שמות י״ב:ל׳

1ויקם פרעה AND PHARAOH AROSE from his bed,

2לילה AT NIGHT and not, as is the way of kings, at three hours in the day (9 o’clock in the morning) (Mekhilta; cf. Berakhot 9b).

3הוא HE, first — and afterwards עבדיו HIS SERVANTS — this informs us that he went round to the houses of his servants and made them get up (Mekhilta).

4כי אין בית אשר אין שם מת FOR THERE WAS NOT A HOUSE WHERE THERE WAS NOT ONE DEAD — If there was a firstborn there, he died: if there was no firstborn there, then the chief person in the house died because the chief person in the house is called בכור, as it is said, (Psalms 89:28) “I also will appoint him to be a בכור, [the highest of the kings of the earth]” (Mekhilta DeRashbi 12:30; cf. Rashi on 4:22; Pesachim Rabbah, Midr. Vajosha). Another explanation is: the Egyptian women were unfaithful to their husbands and bore children from young men, unmarried, and thus they (the Egyptian men) had many firstborn sons sometimes there were five to one woman, each being the firstborn to his own father (thus there might well be no house in which there was not a firstborn) (cf. Mekhilta).