Is shortening the prayer on a journey obligatory, and on what journey?

Shortening the four-unit prayers to two on a journey is one of the widest areas of disagreement on prayer; Ibn Hazm sets it out in the Book of Prayer of al-Muhalla in three questions: the ruling of shortening and on which journeys it applies, its minimum distance, and the length of stay that ends it. ^{1 2 3}

Ibn Hazm lays down that praying the prayers as two units on a journey is obligatory, whether the journey is one of obedience, of disobedience or neither, in safety or in fear; whoever deliberately prays four knowing that it is not allowed has a void prayer, while one who does so by mistake makes the prostration of forgetfulness after the greeting; he holds that shortening each prayer to a single unit in fear on a journey is permitted. ¹ He reports that Abu Hanifa said shortening is obligatory on every journey of obedience or disobedience, so that one who prays four without sitting after two units for the length of the testimony has a void prayer; that Malik said one who prays four on a journey repeats within the time; that al-Shafi'i said shortening is permitted and one may pray four; that Malik and al-Shafi'i allow shortening only on a permitted journey; and that none of the three allowed shortening to one unit in fear. ¹

He argues from the report of 'A'isha that prayer was prescribed as two units, then the Prophet ﷺ emigrated and it was prescribed as four, and the travel prayer was left as it first was, which he gives through several chains from 'Urwa from her; and from 'Umar's words: the prayer of the sacrifice is two units, of the fast-breaking two, of Friday two, and of the traveller two, complete and not shortened, on the tongue of your Prophet. ⁴ He gives Ibn 'Umar's report "the travel prayer is two units; whoever leaves the Sunna has disbelieved", and Ya'la ibn Umayya's report that he said to 'Umar that people were now safe, and 'Umar said: I wondered at what you wonder at and asked the Prophet, and he said, "It is a charity God has given you, so accept His charity." ⁵ He concludes that prayer was prescribed as two units, then raised to four at home after the Hijra while the travel prayer stayed at two, so that whoever goes beyond two has not prayed as commanded; the Prophet did not single out one journey from another, and God's charity, which he ordered to be accepted, may not be refused. ⁶

To those who deny shortening on a journey of disobedience because it is forbidden, Ibn Hazm replies that it is forbidden but still a journey and has the ruling of a journey; they themselves allow dry ablution on it when water is lacking and hold the prayer on it to be obligatory, so there is no difference between what they allow and what they forbid; likewise fornication is forbidden yet requires the full bath as lawful intercourse does, and a highway robber who is wounded prays sitting just as a fighter in God's path does. ⁶ He finds it strange that the Malikis restricted the general word "journey" by their opinion and then allowed carrion to one in dire need even when he is a transgressing highway robber, though the verse excludes him. ⁷

Ibn Hazm notes that Abu Sulayman Dawud and his followers said the prayer is shortened only on the pilgrimage, jihad or the lesser pilgrimage, a view held by a number of early authorities including Ibn Mas'ud, Tawus and Ibrahim al-Taymi, arguing from the verse of al-Nisa' and from the fact that the Prophet shortened only on those journeys; he says that if there were nothing but the verse and the Prophet's practice their view would stand, but his words "two units on a journey" and his order to accept the charity add to the verse and to his practice, and it is not permitted to leave the additional law aside. ⁸ He also reports the Shafi'i argument for choice from the verse's wording negating blame in al-Nisa' 101 and from 'Abd al-Rahman ibn al-Aswad's report from 'A'isha. ⁸

Ibn Hazm holds that whoever leaves the houses of his town, village or place of residence and walks a mile or more prays two units without fail, and if he walks less than a mile prays four. ² He sets out the disagreement of the early authorities: 'Uthman's letter to his governors that the tax collector, the merchant and the estate owner should not shorten, but only one carrying provisions and water; Ibn Mas'ud's words to the people of Kufa, "do not let these farmlands of yours deceive you about your prayer"; Hudhayfa's condition on his companion not to shorten or break the fast between al-Mada'in and Kufa; and the differing reports from Ibn 'Umar about the distance to Khaybar and al-Suwayda'. ^{2 9 10} He reports that Abu Hanifa takes three days' travel and the distance between Kufa and al-Mada'in, that the reports from Malik range from thirty-six to forty-eight miles, and that al-Shafi'i said there is no shortening for less than forty-six miles. ^{11 12 13}

He ends by listing those who shortened over short distances: among the Companions 'Umar, 'Ali, Dihya ibn Khalifa, Ibn Mas'ud, Ibn 'Umar, Anas and Shurahbil ibn al-Simt; among the Successors Sa'id ibn al-Musayyab, al-Sha'bi, Jabir ibn Zayd, al-Qasim ibn Muhammad, Salim ibn 'Abd Allah and others; his own school holds it for a mile or more; and he says he gathered these reports because the Malikis and Shafi'is had begun to claim consensus for their view. ¹⁴ In his view setting the limit in miles rests on no Qur'an, Sunna or consensus, forces its holder into arbitrariness once the limit falls short by a small amount, and the reports from Ibn 'Abbas and Ibn 'Umar differ sharply, the mile figures in them coming from later transmitters. ^{15 16}

Ibn Hazm holds that a traveller on jihad, the pilgrimage, the lesser pilgrimage or any other journey who stays in one place twenty days with their nights or less shortens, whether or not he intended to stay, and if he stays longer by the time of a single prayer he prays in full; this is for prayer only, whereas for fasting, if he stays a day and a night without travelling he must fast. He reports from Ibn 'Umar and Sa'id ibn al-Musayyab that one who resolves to stay fifteen days prays in full, which is Abu Hanifa's view. ³

The schools' views are given only as Ibn Hazm reports them; they have not yet been added from the schools' own books, nor the Hanbali view, nor the details of the single-unit prayer of fear.