The Book of Breathings is the source for Facsimile 1 in the book of Abraham. (with additions or "reconstructions" by Joseph Smith). It is one of a collection of papyri in Joseph Smith's posession when the Book of Abraham was written.
Abraham 1:12 and 1:14 claim that this illustration appears at the start of the book of Abraham.
"And it came to pass that the priests laid violence upon me, that they might slay me also, as they did those virgins upon this altar; and that you may have a knowledge of this altar, I will refer you to the representation at the commencement of this record."
"That you may have an understanding of these gods, I have given you the fashion of them in the figures at the beginning..."
This is pretty clear - if the figure appears at the start of the Book of Abraham then all we have to do is to find the scroll that starts with that figure - the Book of Breathings.
The "Egyptian Alphabet and Grammar" produced by Joseph Smith also directly relates the Book of Abraham to the text of the Book of Breathings. While some have tried to insist that it is solely a creation of Joseph Smith's scribes I would have to ask why they would be doing such a thing - at the very least they must be convinced not only that the Book of Breathings was the original of the Bok of Abraham and that they could correctly relate the two (and how to do that without Joseph Smith ?).