Exode 11:9

But the Lord said to Mosheh, Pharoh will not hearken to you; that I may multiply My wonders in the land of Mizraim.

AND THE ETERNAL SAID UNTO MOSES: PHARAOH WILL NOT HEARKEN UNTO YOU. It was to be expected that Pharaoh and his servants should dread the plague of the firstborn and be in consternation thereof moreso than for anything that had happened to them — indeed they had previously seen all the words of Moses fulfilled. G-d therefore informed Moses that it is He Who is hardening his [Pharaoh’s] heart, so that His wonders would be multiplied through the plague of the firstborn in both man and beast and the judgments He will execute against their gods.
Rashi commented: “So that My wonders may be multiplied. This refers to the slaying of the firstborn, the division of the Red Sea, and the overthrowing of the Egyptians in it.” This explanation is not possible, because of the verse after that which states, And the Eternal hardened Pharaoh’s heart, and he did not let the children of Israel go out of his land., which was mentioned in Verse 9, refers only to those wonders which took place before the exodus, as Verse 10 clearly indicates that on account of the hardening of Pharaoh’s heart he did not let the people go. It thus cannot refer to the division of the Red Sea and the overthrowing of the Egyptians in it, as these wonders happened after the exodus (Mizrachi).

לא ישמע אליכם פרעה. This was a refrain which G’d repeated to Moses and Aaron before every plague except the last one.

ויאמר ה' אל משה לא ישמע אליכם פרעה, G’d told him that the reason why Pharaoh would not react positively to Moses’ warning was simply that He, G’d, had stiffened his heart in order to be able to demonstrate more miracles. This was designed to teach both the Egyptians and the Israelites the multi-faceted power of G’d, as well as His goodness, as He had said: “you will know that I am Hashem.” (10,2) Moses and Aaron performed all these miracles in order to fulfill what G’d had wanted to demonstrate through human messengers. Now, that the time had come to punish the Egyptians, not to demonstrate miracles, and to save the Israelites, i.e. at least the ones that deserved being saved, and also to destroy the Egyptian deities, G’d introduces the part the Passover was to play in this end-game. This portion commences with 12,12-13. G’d wanted that the command-ment of the Passover be communicated to the people by both Moses and Aaron so that just as Moses and Aaron both had had a part to play in their confrontation with Pharaoh, they should also have a similar part to play in getting the Israelites to perform the two commandments which would qualify them to take advantage of Pharaoh’s downfall, i.e. their own salvation, the Exodus. Only in this way would Moses and Aaron reap the reward of all their efforts so far.