מדרש אויף שמות 14:5

(Exodus 14:5) "And it was told to the king of Egypt": Who told him? His emissaries (that he had sent with them.) Some say: He had watchtowers. Others say: Amalek told him.

In the past, Pharaoh's servants said to him (Exodus 10:7) "How long will this one be a stumbling block to us?" and now (Ibid. 14:5) "What is this that we did in sending Israel away from serving us?" They said to him: If we were smitten and we had not sent him away, that would have been enough for us. Or if we were smitten and had sent him away and he had not taken our wealth from us, that would have been enough for us. But we have been smitten, and we have sent him away, and he has taken our wealth from us!

Variantly: "And the heart of Pharaoh was reversed": Scripture hereby apprises us that when Israel left Egypt, its kingdom came to an end, as it is written (Ezekiel 32:19) "Whom have you (Egypt) surpassed in beauty? Descend and be laid to rest with the uncircumcised!"