BibTeX:
@incollection{AbdulBah1915,
  author = {'Abdu'l-Bahá},
  title = {Memorial of Zaynu’l-Muqarrabín},
  booktitle = {Memorials of the Faithful},
  year = {1915},
  publisher = {Bahá'í Publishing Trust},
  url = {https://www.gutenberg.org/cache/epub/19279/pg19279-images.html},
}

APA:
'Abdu'l-Bahá. (1915). Memorial of Zaynu’l-Muqarrabín. In *Memorials of the Faithful*. Bahá'í Publishing Trust.

MLA:
'Abdu'l-Bahá. "Memorial of Zaynu’l-Muqarrabín." *Memorials of the Faithful*, Bahá'í Publishing Trust, 1915.

Chicago:
'Abdu'l-Bahá. "Memorial of Zaynu’l-Muqarrabín." In *Memorials of the Faithful*, Bahá'í Publishing Trust, 1915.