BibTeX:
@incollection{AbdulBah1978,
  author = {'Abdu'l-Bahá},
  title = {187: O thou who art firm in the Covenant! Thy letter},
  booktitle = {Selections from the Writings of 'Abdu'l-Bahá},
  year = {1978},
  url = {https://www.gutenberg.org/cache/epub/19287/pg19287-images.html},
}

APA:
'Abdu'l-Bahá. (1978). 187: O thou who art firm in the Covenant! Thy letter. In *Selections from the Writings of 'Abdu'l-Bahá*.

MLA:
'Abdu'l-Bahá. "187: O thou who art firm in the Covenant! Thy letter." *Selections from the Writings of 'Abdu'l-Bahá*, 1978.

Chicago:
'Abdu'l-Bahá. "187: O thou who art firm in the Covenant! Thy letter." In *Selections from the Writings of 'Abdu'l-Bahá*, 1978.