BibTeX:
@incollection{ShoghiEffendi1944,
  author = {Shoghi Effendi},
  title = {Dayyán, the Third Letter to Believe},
  booktitle = {God Passes By},
  year = {1944},
  publisher = {Bahá'í Publishing Trust},
  url = {https://www.bahai.org/library/authoritative-texts/shoghi-effendi/god-passes-by/},
}

APA:
Shoghi Effendi. (1944). Dayyán, the Third Letter to Believe. In *God Passes By*. Bahá'í Publishing Trust.

MLA:
Shoghi Effendi. "Dayyán, the Third Letter to Believe." *God Passes By*, Bahá'í Publishing Trust, 1944.

Chicago:
Shoghi Effendi. "Dayyán, the Third Letter to Believe." In *God Passes By*, Bahá'í Publishing Trust, 1944.