Fasting (Ar. s áawmor ṣīām, Per. rūza) is mentioned in several verses in the Qurʾān. The prescription for fasting in the month of Ramażān and several of its regulations are outlined in Qurʾān 2:183-85 and 2:187, while other verses prescribe fasting as an alternative penance for certain offenses (4:92; 5:89, 95; 58:4) or as a substitute for specific pilgrimage rites (2:196). Together with the Hadith (e.g., Boḵārī, I, pp. 472-98; Ḥorr ʿĀmelī, X, pp. 1-556), these verses form the basis for the legal regulations governing Islamic fasting. Ṣāʾemūn and ṣāʾemāt (men and women who fast) are mentioned in the Qurʾān (33:35) as two of many epithets of those who will be forgiven and rewarded by God. According to Qurʾān 19:26, the Virgin Mary (Maryam) is told to take a vow of silence that is described as a fast: “And if you see anyone, say: I have made a vow of ṣawm to the Merciful One, and I will speak to no one on this day,” apparently in recognition of the Christian practice of observing silence when fasting (Berg, p. 94).

According to both Sunnite and Shiʿite jurisprudence (feqh, q.v.), fasting requires abstinence (emsāk) from the following actions: ingesting food or drink (whether by mouth or injection), sexual intercourse, and smoking. Shiʿites must also be careful not to attribute false statements to God, the Prophet, and the Imams, or being in a state of major ritual impurity (janāba) during the fasting hours. The fast is considered broken when the above acts are done intentionally; however, some Shiʿite scholars also specify that even unintentionally being in janāba during Ramażān invalidates the fast (e.g., Ḵomeynī, I, p. 282).

Fasting begins at dawn and continues until night (Qurʾān 2:187). Sunnite feqh recommends that the faster hasten to break his fast as soon as the sun has set, while according to a well-known opinion in Shiʿite feqh, the faster must wait until all redness disappears from the eastern half of the sky (Ḥellī, I, p. 61). It is preferable that the meal breaking the fast (foṭūr or efṭār) begin with an odd number of dates in accordance with the Sunna, and that the pre-fast meal (soḥūr) be taken as late as possible in the second half of the night (Jazīrī, I, p. 577).

Sunnite and Shiʿite feqh recognize several types of fasting: Ramażān, kaffāra (compensation), qażā (making up), eʿtekāf (seclusion in the mosque), and manḏūr (vowed fasts). For each, the faster must make a special formulation of his intention (nīya). Fasting in the month of Ramażān is one of the pillars of Islam and is obligatory on Muslims who are mature (bāleḡ), sane (ʿāqel), and physically able.

Kaffāra fasts are imposed when one is unable to pay the compensation normally required in cases of accidental killing (Qurʾān 4:92), breaking an oath (Qurʾān 5:89), hunting during pilgrimage (Qurʾān 5:95), and resuming a marriage dissolved by pronouncing the ẓehār formula (Qurʾān 58:4), combining the major (ḥajj) and minor (ʿomra), or prematurely shaving the head during the pilgrimage (Qurʾān 2:196). Kaffāra is required for intentionally breaking a Ramażān fast and consists of either freeing a slave, feeding sixty needy people, or fasting for two consecutive months. Shafiʿites impose kaffāra only when the fast is broken by sexual intercourse; Hanafites and Shiʿites include ingesting food or drink; and Shiʿites add remaining in janāba. Shiʿite feqh also requires that those who break the fast with a prohibited substance (such as alcohol) must undertake all three of the Ramażān kaffārāt (Maḡnīya, pp. 154-55).

Qażāʾ fasts are required from those who break an obligatory fast or who are excused from part of all of the Ramażān fast. The latter include travelers, those who are ill, pregnant or nursing women, women experiencing menstruation or post-partum bleeding, the elderly, and any others for whom fasting is a hardship or impossible. The number of days missed during a given Ramażān must be made up before the subsequent Ramażān; those unable to do so (e.g., the chronically ill and the elderly) may feed a needy person for each day missed (Qurʾān 2:184).

Voluntary fasts may be undertaken when all obligatory fasts have been fulfilled. A specific nīya is not necessary, and if the fast is broken, it does not need to be made up. Certain days are recommended (mandūb) for fasting, such as three days of every month, particularly the “white days” (13, 14, and 15) and the two months preceding Ramażān (i.e., Rajab and Šaʿbān). Shiʿites add their particular sacred days, such as Ḡadīr (q.v.) and Mobāhala (Ḥellī, I, p. 208; Schmucker, p. 276) .

All types of Fasting are proscribed (ḥarām) for women experiencing menstruation or post-partum bleeding, and for all Muslims on the Feast of Breaking the Fast (ʿĪd al-feṭr) which marks the end of Ramażān; the Feast of Sacrifice (ʿĪd al-ażḥā); and the the three days after the ʿĪd al-ażḥā (ayyām al-tašrīk). In addition, a woman may not undertake a voluntary fast without the permission of her husband (Maḡnīya, pp. 160-61.)

Fasting is valued as a method of increasing self-restraint and willpower, as well as an experience which generates feelings of compassion for the poor (Ṭabbāra, pp. 165-67). The communal nature of the Ramażān fast very likely makes it the most widely observed pillar of Islam (Gardet, p. 127).