The Maxim of Custom and Its Role in Contracts and Marriage

The fifth and final foundational maxim in Al-Ashbah wan-Naza'ir — 'custom is authoritative' (al-'adah muhakkamah) — reflects one of the most distinctive features of Hanafi jurisprudence: its systematic recognition of local custom and social practice ('urf and 'adah) as legitimate sources for legal determination. Ibn Nujaym traces this principle to the Quranic command to 'take what is given freely and command what is customarily right and turn away from the ignorant' (al-A'raf 7:199), and to the broad Hanafi principle that established social practice operates as a form of implicit agreement among people.

Ibn Nujaym distinguishes carefully between 'urf sahih (sound custom) and 'urf fasid (corrupt custom). Sound custom is one that does not contradict a clear text of the Quran or an authenticated hadith and does not violate a recognized objective of Islamic law. Only sound custom is legally authoritative; corrupt custom — such as usurious lending practices that have become customary in a society — acquires no legal recognition merely by being widespread.

In the law of contracts, the custom maxim has far-reaching consequences. The Hanafi school recognizes bay' al-ta'ati (contract by mutual exchange without explicit verbal offer and acceptance) as valid when local custom accepts it — allowing a person to buy a newspaper, take food from a street vendor, or purchase goods from a vending machine without a formal verbal offer and acceptance. Ibn Nujaym explains that the conditions of a contract may be supplied by custom even when not explicitly stated: if it is the custom that a hired laborer works certain hours and receives meals, those terms are implied in the hire without being spelled out.

In the law of marriage, the custom maxim plays a prominent role in determining the customary dower (mahr al-mithl) when no specific mahr was agreed upon, or when an unusual mahr was agreed that departs dramatically from custom. The Hanafi school holds that the mahr al-mithl is calculated by reference to the dower given to the bride's female relatives of comparable status from her father's family — this 'comparable status' itself determined by local custom regarding wealth, lineage, and social position.

The maintenance obligation (nafaqah) of a husband toward his wife is also partly determined by custom. The minimum level of maintenance is the amount sufficient for the wife's needs according to the custom of her social class, not an absolute uniform standard. Ibn Nujaym examines how this operates in practice: a wife from a wealthy family receives a higher nafaqah than one from a modest background, not merely because of her own preference but because custom sets the expectation for each social level.

In commercial law, the custom maxim allows the Hanafi school to validate a range of contracts and financial arrangements that would otherwise be questioned on technical grounds. The forward delivery contract (salam) is valid by established custom and prophetic permission even though it involves payment for goods not yet in existence — a feature that would otherwise run afoul of the general prohibition on gharar (uncertainty). Custom-based necessity permits it because of the widespread agricultural and commercial need it serves.

Ibn Nujaym concludes his treatment of this maxim with an important caution: custom can change, and legal rulings grounded in custom may change with it. A ruling that was given in a particular city based on the customs of that time and place may not automatically transfer to a different city or a later era where customs differ. This makes the custom maxim both a powerful legal tool and a source of genuine intellectual humility: the jurist must always ask not only what the rule is but what custom the rule was based upon, and whether that custom still obtains.