The fourth pillar of Islam is sawm, or fasting. Clear Qur'anic references to fasting account for the early introduction of this ritual practice. The Qur'an prescribes fasting during the month of Ramadan, the 9th month of the 12-month Islamic lunar year (see Calendar). The month of Ramadan is sacred because the first revelation of the Qur'an is said to have occurred during this month. By tradition the month starts with the sighting of the new moon by at least two Muslims. For the entire month, Muslims must fast from daybreak to sunset by refraining from eating, drinking, and sexual intercourse. Menstruating women, travelers, and sick people are exempted from fasting but have to make up the days they miss at a later date.

According to various traditional interpretations, the fast introduces physical and spiritual discipline, serves to remind the rich of the misfortunes of the poor, and fosters, through this rigorous act of worship, a sense of solidarity and mutual care among Muslims of all social backgrounds. Thus Muslims usually engage in further acts of worship beyond the ordinary during Ramadan, such as voluntary night prayer, reading sections from the Qur'an, and paying voluntary charity to the poor. Muslims may even choose to wake before daybreak to eat a meal that will sustain them until sunset. After the fasting ends, the holiday of breaking the fast, 'id al-fitr, begins, lasting for three days.

At any time of year fasting is also required as a compensation for various offenses and violations of the law. Many Muslims also perform voluntary fasts at various times of the year as acts of devotion and spiritual discipline. However, such additional fasting is not required by Islamic law.