BibTeX:
@incollection{AbdulBah1922,
  author = {'Abdu'l-Bahá},
  title = {Leaves of One Tree: 'Abdu'l-Bahá at the Orient-Occident Conference},
  booktitle = {The Promulgation of Universal Peace},
  year = {1922},
  publisher = {Bahá'í Publishing Trust},
  url = {https://www.bahai.org/library/authoritative-texts/abdul-baha/promulgation-universal-peace/},
}

APA:
'Abdu'l-Bahá. (1922). Leaves of One Tree: 'Abdu'l-Bahá at the Orient-Occident Conference. In *The Promulgation of Universal Peace*. Bahá'í Publishing Trust.

MLA:
'Abdu'l-Bahá. "Leaves of One Tree: 'Abdu'l-Bahá at the Orient-Occident Conference." *The Promulgation of Universal Peace*, Bahá'í Publishing Trust, 1922.

Chicago:
'Abdu'l-Bahá. "Leaves of One Tree: 'Abdu'l-Bahá at the Orient-Occident Conference." In *The Promulgation of Universal Peace*, Bahá'í Publishing Trust, 1922.