Jeff Lindsay explains:
Facsimile 1 in the Book of Abraham shows Abraham on a pagan altar about to be sacrificed by an idolatrous priest, and chapter 1 relates the story of how Abraham was seized, bound, and put on an altar to be sacrificed by a pagan priest, who had previously sacrificed three virgins on the same altar because they refused to worship idols.
All this is wildly innovative, based on what Joseph Smith could have known about Abraham, but it fits well with numerous ancient traditions about Abraham. In fact, so many ancient texts refer to one or more attempts to the sacrifice Abraham that one can wonder why the Bible is lacking that detail. A majority of these texts indicate that the attempted sacrifice was made by throwing Abraham into fire, from which he was delivered by God’s power. Though the Book of Abraham does not say that Abraham or sacrificed victims were thrown into a fire, ancient animal sacrifices typically involved killing the animal and then burning the victim, and the same may have applied to the human sacrifices mentioned in the Book of Abraham. On the other hand, since the name of the Chaldean city, “Ur,” also means fire in Hebrew, perhaps some writers have assumed that Abraham’s escape from Ur of the Chaldeans was deliverance from fire, possibly blending fire into the story of his deliverance from sacrifice.– Jeff Lindsay – Questions about the Book of Abraham, Part 3: Does It Agree with other Ancient Texts?
Where did Joseph Smith get the idea that Abraham was going to be sacrificed?