BibTeX:
@incollection{BahyyihKhnum1982,
  author = {Bahíyyih Khánum},
  title = {Now Is a Period of Great Tests},
  booktitle = {Bahíyyih Khánum: The Greatest Holy Leaf},
  year = {1982},
  publisher = {Bahá'í World Centre},
  url = {https://www.gutenberg.org/cache/epub/19242/pg19242-images.html},
}

APA:
Bahíyyih Khánum. (1982). Now Is a Period of Great Tests. In *Bahíyyih Khánum: The Greatest Holy Leaf*. Bahá'í World Centre.

MLA:
Bahíyyih Khánum. "Now Is a Period of Great Tests." *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.

Chicago:
Bahíyyih Khánum. "Now Is a Period of Great Tests." In *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.