Why did Moshe warn Yisrael not to leave their houses until the morning even though Hashem had not instructed him to tell them this?
(12,22) “And you shall take a bunch of hyssop and dip it in the blood which is in the basin, and touch the lintel and the two doorposts with some of the blood which is in the basin. And you shall not go out, anyone, from the door of his house until morning.”
Earlier when Hashem gave instructions to Moshe and Aharon, He told them that the blood should be placed first on the two doorposts and then the lintel, and He did not tell them that they should not go out from their houses until the morning. But here when Moshe is giving instructions to Yisrael, he said the opposite - “and you shall touch the lintel and the two doorposts with some of the blood which is in the basin”, and he also told them that “you shall not go out, anyone, from the entrance of his house until morning”!
One explanation which I heard is that Yisrael’s redemption from Egypt was not due to their own merit, but due to the merit of Moshe and Aharon who were equal to Yisrael. And although the merit of the forefathers also contributed to their redemption, their main merit was that of Moshe and Aharon who were of their generation, and the merit of the forefathers from the previous generations was secondary. Also, it is well known that only someone who is saved due to his own merit is permitted to see the downfall of the wicked.
Therefore Hashem first mentioned the doorposts and then the lintel to convey the message that Yisrael’s main merit came from Moshe and Aharon who are compared to two doorposts, and the merit of the forefathers, who are compared to the lintel which is above the doorposts since they preceeded Moshe and Aharon by many years, was only an auxiliary merit. And since their main merit came from Moshe and Aharon it was not necessary to warn them not to leave their houses until the morning, because they already knew from the incident of S’dom that the angels warned Lot not to look behind him since his salvation was not due to his own merit but that of Avrohom, and therefore he was not permitted to see the downfall of S‘dom. And his wife who did look was punished. Thus since Yisrael’s salvation was in the merit of Moshe and Aharon they would realize by themselves that they were not permitted to leave their houses to see the downfall of the Egyptians with the death of their firstborn.
But Moshe due to his great humility attributed Yisrael’s salvation to the merit of the forefathers and considered his and Aharon’s merit to be secondary, and thus he mentioned the lintel before the doorposts. Because of this he was afraid that when they hear that their main merit comes from the forefathers which could be considered as their own merit since they are their offspring, they might think that they are permitted to go out from their houses to see the downfall of the Egyptians. Therefore it was necessary to warn them not to leave their houses until morning, because once permission is given to the destroyer to destroy he does not distinguish between good and evil. Therefore, they must stay in their houses so that the blood on the lintel and the doorposts will protect them from suffering the same fate as the Egyptians.