- 2014. Houtsma, M.T.; Wensinck, A.J.; Lévi-Provençal, E.; Gibb, H.A.R.;
Heffening, W., eds. (1993). E.J. Brill's
First Encyclopaedia of Islam, 1913–1936...
- "Al-Shafi". The
Encyclopedia Of Islam. Vol. IX. Brill. pp. 182–183.
Heffening, W. (1934). "Al-Shafi'i". The
Encyclopaedia of Islam. Vol. IV. E. J. Brill...
- 84–85. Brill, 2022. Houtsma, M. Th.; Wensinck, A. J.; Arnold, T. W.;
Heffening, W.; Lévi-Provençal, E., eds. (1993). "Ḳawah".
First Encyclopedia of Islam...
- 'Ali b. Abï Ṭālib". In Houtsma, M. Th.; Wesinck, A. J.; Arnold, T. W.;
Heffening, W.; Lévi-Provençal, E. (eds.). The Encyclopædia of
Islam - A Dictionary...
- Houtsma, M. Th.; Wensinck, A. J.; Levi-Provençal, E.; Gibb, H. A. R.;
Heffening, W. (eds.). E.J. Brill's
First Encyclopaedia of Islam, 1913–1936, Volume...
- 'Ali b. Abï Ṭālib". In Houtsma, M. Th.; Wesinck, A. J.; Arnold, T. W.;
Heffening, W.; Lévi-Provençal, E. (eds.). The Encyclopædia of Islam: A Dictionary...
- "Yazīd b. Mu'āwiya". In Houtsma, M. Th.; Wensinck, A. J.; Gibb, H. A. R.;
Heffening, W.; Lévi-Provençal, E. (eds.). The Encyclopædia of Islam: A Dictionary...
- Ratnagotravibhaga. SUNY Press. ISBN 0791403572. Al-Azhar, Encyclopædia
Britannica Heffening, W. (2012), "Murtadd."
Encyclopaedia of Islam,
Second Edition. Edited...
- In Houtsma, Th.; Wensinck, A. J.; Lévi-Provençal, E.; Gibb, H. A. R.;
Heffening, W. (eds.). E.J. Brill's
First Encyclopaedia of Islam, 1913-1936, Volume...
- ("authentic")
Hadith rather than the Quran.
Writing in the
Encyclopedia of Islam,
Heffening holds that
contrary to the Quran, "in
traditions [i.e. hadith], there...