BibTeX:
@incollection{Bahullh1982,
  author = {Bahá'u'lláh},
  title = {O My Leaf! Hearken Thou unto My Voice},
  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:
Bahá'u'lláh. (1982). O My Leaf! Hearken Thou unto My Voice. In *Bahíyyih Khánum: The Greatest Holy Leaf*. Bahá'í World Centre.

MLA:
Bahá'u'lláh. "O My Leaf! Hearken Thou unto My Voice." *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.

Chicago:
Bahá'u'lláh. "O My Leaf! Hearken Thou unto My Voice." In *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.