Isaiah 19:19-20;
"In that day there will prove to be an altar to Jehovah in the midst of the land of Egypt, and a pillar to Jehovah beside its boundary. 20 And it must prove to be for a sign and for a witness to Jehovah of armies in the land of Egypt; for they will cry out to Jehovah because of the oppressors, and he will send them a savior, even a grand one, who will actually deliver them."
Russell thought Isaiah 19 was referring to the great Pyramid, though it is strange that it is not more direct reference to it.