Exodus 8:26
And Moses said, It is not meet so to do; for we shall sacrifice the abomination of the Egyptians to the LORD our God: lo, shall we sacrifice the abomination of the Egyptians before their eyes, and will they not stone us? And Moses said, It is not meet so to do; for we shall sacrifice the abomination of the Egyptians to the LORD our God: lo, shall we sacrifice the abomination of the Egyptians before their eyes, and will they not stone us?
Pharaoh offered to let Israel sacrifice to God while staying inside Egypt, and Moses refused that offer. The abomination of the Egyptians means the animals Israel was required to sacrifice, which the Egyptians regarded as sacred and worshiped as sacred beasts and objects of worship. Egyptian reverence for these creatures was so great that to kill one, even accidentally, was an unpardonable crime and unpardonable offense punishable with the offender's death and to be answered for with the offender's life. To sacrifice such animals before the eyes of the Egyptians and in their sight in the land would provoke them to stone the Israelites, so Israel could not worship in Egypt without giving offense to their masters.
For that reason acceptable worship while remaining in Egypt was impossible, and the sacrifice had to take place outside Egypt in the wilderness. The separation God required and commanded was a three days' journey into the wilderness to sacrifice as commanded. Pharaoh's proposal was only a limited concession that did not satisfy the demand to let Israel go.