Midrash su Salmi 89:28

(Ibid., cont.:) BECAUSE THERE WAS A GREAT CRY…. How so? When an Egyptian marries five women, he has five first-born from them; and likewise when a woman is married to five men, she has from them five first-born {from them}. So all of them died. It was in order to confirm what is stated (in Exod. 11:5): AND EVERY FIRST-BORN <IN THE LAND OF EGYPT> SHALL DIE. Moreover, a house in which there was no first-born had an outcry {greater than the house} [as great as in the house] in which there was a first-born, because the master of the house was <himself> a first-born,⁷⁶I.e., by virtue of his status the head of the house was considered a first-born. as stated (in Ps. 89:28 [27]): I WILL ALSO APPOINT HIM FIRST-BORN. Thus it is stated (in Exod. 12:30): BECAUSE THERE WAS A GREAT CRY IN EGYPT; FOR THERE WAS NO HOUSE WHERE THERE WAS NOT SOMEONE DEAD.