BibTeX:
@incollection{AbdulBah1915,
  author = {'Abdu'l-Bahá},
  title = {After the Great Separation: The Believers Left Behind in Baghdád},
  booktitle = {Memorials of the Faithful},
  year = {1915},
  publisher = {Bahá'í Publishing Trust},
  url = {https://www.bahai.org/library/authoritative-texts/abdul-baha/memorials-faithful/},
}

APA:
'Abdu'l-Bahá. (1915). After the Great Separation: The Believers Left Behind in Baghdád. In *Memorials of the Faithful*. Bahá'í Publishing Trust.

MLA:
'Abdu'l-Bahá. "After the Great Separation: The Believers Left Behind in Baghdád." *Memorials of the Faithful*, Bahá'í Publishing Trust, 1915.

Chicago:
'Abdu'l-Bahá. "After the Great Separation: The Believers Left Behind in Baghdád." In *Memorials of the Faithful*, Bahá'í Publishing Trust, 1915.