פירוש על שמות 10:1

ויאמר ה' אל משה בא אל פרעה AND THE LORD SAID UNTO MOSES GO IN UNTO PHARAOH and warn him.

AND THE ETERNAL SAID UNTO MOSES: GO IN UNTO PHARAOH; FOR I HAVE HARDENED HIS HEART AND THE HEART OF HIS SERVANTS. The Holy One, blessed be He, informed Moses that it is He Who has hardened their hearts in spite of their fear of Him during the hail and their confession of sin.¹Although Scripture above (9:27) mentions only Pharaoh confessing his sin, it is apparent from Verse 30 there that the king’s servants also made this confession, since Moses said to all of them, But as for thee and thy servants, I know that ye will not yet fear the Eternal G-d. This is a clear indication that the servants too had joined the king in admitting their guilt. Ramban is thus correct in writing here in the plural: “their confession of sin.” And He explained to him: “The reason I hardened their hearts is that I might set in their midst these signs that I wish to do among them so that the Egyptians will know My power, but not in order that I can punish them more on account of this hardening of heart, and also that you and all Israel should recount during the coming generations the power of My deeds, and you shall know that I am the Eternal,²Verse 2. and whatsoever I please, I do in heaven and in earth.”³See Psalms 135:6.

כי אני הכבדתי את לבו, during all the preceding plagues we do not find that G’d had told Moses that it was He Who had stiffened Pharaoh’s heart. However, since we have reached the stage where Pharaoh himself had said that “G’d is just whereas he and his people are the sinners,” (9,27) and still he had reneged and sinned deliberately, a phenomenon which must have seemed incomprehensible to Moses, G’d explains the psychology behind this, i.e. that it was not as hard to understand, as He Himself had to stiffen Pharaoh’s resolve causing him to renege. ואת לב עבדיו, as we are told in 9,34.