Book 5 & 6;
In the Mysteries of Charity and Its Important Elements (Kitāb asrār al‑zakāt wa-muhimātiha; book 5), and the Mysteries of Fasting and Its Important Elements (Kitāb asrār al-ṣiyyām wa-muhimātiha; book 6), of the forty books of the Revival of the Religious Sciences (Iḥyāʾ ʿulūm al-dīn), Abū Ḥāmid al-Ghazālī explains the inner and outer meanings of charity and fasting, two pillars of worship for Muslims. These are two of the five books that deal with the inner dimensions of Islamic worship; books 3, 4, and 7 address purification, prayer, and pilgrimage, respectively.
In book 5 al-Ghazālī begins with a discussion of the various kinds of zakāt and the conditions under which they become obligatory. He explains the manners related to zakāt and its inward and outward conditions. He then discusses the recipients of zakāt, who qualifies for it and in what circumstances, and the manners related to receiving it. Finally, al-Ghazālī explains charity and its virtues.
In book 6 al-Ghazālī begins with the outward obligations and sunnah elements of fasting, and what is required if the fast is broken. He explains the mysteries of fasting and its inner conditions. Finally, he discusses the voluntary fasts and their order in supererogatory worship