BibTeX:
@incollection{theBb1976,
  author = {the Báb},
  title = {O People of the Bayán! Be on Your Guard},
  booktitle = {Selections from the Writings of the Báb},
  year = {1976},
  url = {https://www.gutenberg.org/cache/epub/18828/pg18828-images.html},
}

APA:
the Báb. (1976). O People of the Bayán! Be on Your Guard. In *Selections from the Writings of the Báb*.

MLA:
the Báb. "O People of the Bayán! Be on Your Guard." *Selections from the Writings of the Báb*, 1976.

Chicago:
the Báb. "O People of the Bayán! Be on Your Guard." In *Selections from the Writings of the Báb*, 1976.