BibTeX:
@incollection{AbdulBah,
  author = {'Abdu'l-Bahá},
  title = {The Third Step},
  booktitle = {'Abdu'l-Bahá in London},
}

APA:
'Abdu'l-Bahá. The Third Step. In *'Abdu'l-Bahá in London*.

MLA:
'Abdu'l-Bahá. "The Third Step." *'Abdu'l-Bahá in London*, n.d..

Chicago:
'Abdu'l-Bahá. "The Third Step." In *'Abdu'l-Bahá in London*, n.d..