During the confrontation with Paro Moshe repeats God's demand several times: let my people go that they may serve Me -- שַׁלַּח עַמִּי וְיַעַבְדֻנִי . During the plague of frogs Paro makes this offer (Shemot 7:4):
. וַיִּקְרָא פַרְעֹה לְמשֶׁה וּלְאַהֲרֹן וַיֹּאמֶר הַעְתִּירוּ אֶל יְהֹוָה וְיָסֵר הַצְפַרְדְּעִים מִמֶּנִּי וּמֵעַמִּי וַאֲשַׁלְּחָה אֶת הָעָם וְיִזְבְּחוּ לַיהֹוָה:
Thereupon, Pharaoh summoned Moses and Aaron, and said, "Entreat the Lord that He remove the frogs from me and from my people, and I will let out the people [of Israel] so that they may sacrifice to the Lord."
This morning I had the chance to ask somebody who's spent a fair bit of time studying ancient cultures if animal sacrifice was part of Egyptian religious practice at this time. He said it was not (humans yes, animals no). I didn't grill him for sources. (But Google doesn't seem to disagree with him, from what I've found.)
Moshe never said anything about the form of the divine service for which the people needed to be released. Where did Paro get the idea that it would be sacrifices and not something else?
Rashi has nothing to say on this, nor did any of the miscellaneous chumashim we had on hand.