Al-Ghazali: The Mysteries of Purification. 
Book 3 of the Ihya ulum al-din. The Revival of The Religious Sciences

Abu Hamid Al-Ghazali

In The Mysteries of Purification (Kitāb asrār al-ṭahāra), the third of the forty books of the Revival of the Religious Sciences (Iḥyāʾ ʿulūm al‐dīn), Abū Ḥāmid al-Ghazālī explains the fundamentals of the purification that is necessary in order to perform the five daily prayers. The book begins with an introduction to the general topic of purity. Al-Ghazālī explains the ḥadīth “Purification is half of faith,” and reminds readers that, for the earliest Muslims, inner purification was much more important than outer purification.

In chapter 1, he considers purification from impurities: What are the impurities and how should they be removed? Over the following chapters, al-Ghazālī examines the lesser ablution (wuḍūʾ), the greater ablution (ghusl), and the “dry ablution” (ablution without water, or tayammum). He discusses the etiquette of the bathhouse and outlines exactly how to clean the body of external things and natural growths; finally, he discusses the beard and practices related to it.