וַֽיֶּחֱזַק֙ לֵ֣ב פַּרְעֹ֔ה וְלֹ֥א שִׁלַּ֖ח אֶת־בְּנֵ֣י יִשְׂרָאֵ֑ל כַּאֲשֶׁ֛ר דִּבֶּ֥ר יְהוָ֖ה בְּיַד־מֹשֶֽׁה׃ (פ)
И сердце фараона ожесточилось, и он не отпустил сынов Израилевых; как сказал Господь Моисею.
Or HaChaim
ויאמר ה׳ אל משה, G'd said to Moses, etc. The Torah mentions אמירה, the soft approach as well as G'd's name as the Merciful One. This did not refer to the "sender," i.e. to G'd, but to the messenger Moses, as we see in verse 3. [Whereas as recently as 9,35 the Torah referred to Moses as having spoken sternly, דבר, to Pharaoh, now there is a change of mood. Ed.] Moses spoke in a kindly manner to Pharaoh even while delivering a warning of a devastating plague. G'd warned him to do so as otherwise he might have lived up to the description of the righteous man in Psalms 58,11 where the Holy Spirit describes the righteous as rejoicing when he observes G'd taking revenge. Moses is reminded that what truly makes the righteous happy is seeing G'd's attribute of Mercy in action.
Chizkuni
ואני אקשה, “and I (G-d) will toughen;” Rashi points out that during the first five plagues G-d actually did not yet harden Pharaoh’s heart, and the Torah only speaks about vayechezak lev paroh, meaning that Pharaoh “remained obstinate without any help from G-d.” When the Torah writes during the seventh plague, the plague of hail in chapter 9 verse 35, that vayechezak lev paroh, that “Pharaoh’s heart remained obstinate,” which appears to contradict what Rashi said, seeing that line occurs during the seventh plague, this line appears after in verse 34, G-d had already hardened his heart. In that instance, for the first time, Pharaoh had admitted being guilty, and after G-d had hardened his heart he was able to hold out still longer in his obstinate ways. In chapter 10,1, G-d explains to Moses that Pharaoh’s behaviour from then on was understandable only because He had already hardened his heart. He was already no longer a free agent. The reason is simple, as Rashi explains. Once a person has remained obstinate in the face of five plagues clearly orchestrated by G-d, he is deprived of his most precious attribute, that of freedom of choice. Even if Pharaoh had freed the Israelites at that stage, G-d would not have let him, until He had been able to demonstrate Hjs powers completely.