Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.108.

Section : Types of Fasting.

There are two types of fasting: obligatory and voluntary. Obligatory can be further subdivided into the fast of Ramadan, the fast of expiation and the fast of fulfilling a vow. Here we shall discuss the Ramadan and voluntary fasts.