The Concise Oxford Dictionary of World Religions (145 page)

(
c.
781–857 (AH 165–243)).
Sh
fi‘ite
theologian who turned to ascetic renunciation and moral purification, and is regarded as the first
Sunni
mystic (
S
f
) to organize a theologically systematic approach to God. He was a prolific writer, with about 200 works ascribed to him. His
Kit
b al-ri‘
ya li-
uq
q Allah
(The Book of Observance of that which is Owed to God) is a book of spiritual advice, which includes the importance of self-examination (Arab.,
mu
saba
), hence his name.