The Islamic duties (ibadat)are divisible into three categories, depending on a duty's nature whether it mainly involves bodily acts or financial expenditure.
1. The purely bodily ibadat are those which, like fasting (sawm)and prayer (salat), do not involve any financial aspect. According to the four Sunni schools, such duties cannot in any circumstance be delegated to a proxy (na'ib), either on behalf of a living or a dead person. But according to the Imamiyyah school, taking a na'ibispermissible on behalf of a dead person, though not for a living person, to perform sawm and salat for him, and under all circumstances.
2. The purely financial ibadat are those which do not involve bodily acts, such as khums and zakat. In such ibadat, all legal schools agree, it is permissible to take a na'ib. It is permissible for one to depute another to take out zakat and pay other kind of alms (sadaqat)from his assets.
3. The duties which involve both bodily and financial aspects, such as the Hajj, which requires such bodily acts as tawaf (circumambulation of the Ka'bah), say (to and fro movement between Marwah and Safa), ramy (the symbolic throwing of stones), and financial expenditures such as for the journey and its accompanying requirements. All the five legal schools agree that one who is capable of undertaking the Hajj in person and fulfils all the conditions thereof, should do so himself in person. It is not permissible for him to depute another to undertake it, and if he does so it would not relieve him of his obligation to perform it himself. If he does not do it in his life, according to the Shafi'i, Hanbali and Imamiyyah schools, he is not relieved of the duty because of the preponderance of the financial aspect, and it is obligatory to hire someone to perform the Hajj with a similar expenditure. In case he does not make a will for the Hajj, the amount should be taken out from his undivided heritage. [13]
According to the Hanafi and Maliki schools, he is relieved of the obligation due to the bodily aspect; but if he mentions it in his will, the expense is taken out from the one third of his inheritance‑‑like all other bequests‑‑and if he doesn't, istinabah isnot obligatory.
 
The Physically Incapable (al‑Qadir al‑Ajiz)
One who meets all the financial conditions for the Hajj pilgrimage but is incapable of undertaking it personally due to old age or some incurable disease, all the legal schools agree, is relieved of the obligation of performing the Hajj in person, for God says:  (... and He has laid no impediment in your religion ....). [14] However, it is obligatory upon him to hire someone to perform the Hajj for him. But if he doesn't, is it a negligence of a duty whose fulfilment continues to remain upon him? All the legal schools, with the exception of the Maliki, agree that it is obligatory upon him to hire someone to perform the Hajj for him. The Maliki says that the Hajj is not obligatory upon one who is incapable of undertaking it in person. (al‑Mughni, al‑Tadhkirah)
Furthermore, if a sick person recovers after deputing someone to perform his Hajj, is it obligatory upon him on recovery to perform the Hajj in person? According to the Hanbali school, another Hajj is not obligatory. But according to the Imamiyyah, Shafi'i and Hanafi schools it is obligatory, because what was fulfilled was the financial obligation, and the bodily obligation has remained unfulfilled.
 
Istinabah in al‑Hajj al‑Mustahabb
According to the Imamiyyah and Hanafi legal schools; one who has performed the Hijjat al‑'Islam, if he wants to depute another for a voluntary, mustahabbHajj, may do so, even if he is capable of undertaking it in person. But according to the Shafii school, it is not permissible. There are two narrations from Ahmad ibn Hanbal, one indicating prohibition and the other permission. According to the Maliki school, it is permissible for an incurable sick person and for one who has performed the obligatory Hajj to hire another for the Hajj. The Hajj so performed is valid, though makruh (reprehensible). It is not considered as the Hajj of the hirer (mustajir)and is counted as the mustahabbHajj of the hired (ajir). The hirer gets the reward for providing assistance in the performance of the Hajj and shares the blessings of the prayers offered. When the Hajj is performed for the benefit of a dead person, irrespective of whether he has asked for it in his will or not, it is counted neither as fulfilment of the duty (fard) nor as a supererogatory (nafl)act, nor does it relieve him of the duty of the obligatory Hajj. (al‑Fiqh ala al‑madhahib al‑'arbaah).
 
The Conditions for the Na'ib
The na'ib should fulfil the conditions of: bulugh (adulthood), aql (sanity), belief in Islam, exemption from the duty of obligatory Hajj, and ability to perform the Hajj properly. A man may represent a woman and a‑woman may represent a man, even if both the na'ib and the one whom he represents have not performed the Hajj before. [15]
Should the na'ib commence the journey from his own place or that of the deceased whom he represents, or from one of the mawaqit?According to the Hanafi and Maliki schools, the na'ib should commence the pilgrimage journey from the place of the deceased, if he has not specified the starting point; otherwise according to his wish. According to the Shafii school, the pilgrimage commences from one of the mawaqit; if the deceased person has specified one, then the na'ib must act accordingly, otherwise he is free to choose one of the mawaqit. According to the Hanbali school, the na'ib must start from the place that the deceased was obliged to begin from if he had performed the Hajj himself, and not from the place of his death. If the deceased person had attained istita'ah at a place to which he had migrated, later returning to his own place, the na'ib should start from the place of migration, not from the deceased person's home, except when the distance (between his home‑town and the place of migration) is less than what is required for qasr in prayers performed by a traveller.  [16]
According to the Imamiyyah school, the Hajj is classified into miqati (i.e. one which starts from one of the mawaqit)and baladi (i.e. one which starts from the town of the deceased). If the deceased has specified one of these two kinds, then the one specified. If he has not specified, any one of the two may be performed. Otherwise the Hajj is miqati and, if possible, starts from the miqat nearest to Mecca, or else the miqat nearest to the town of the deceased. The cost of al‑Hijjat al‑miqatiyyah istaken out from the undivided legacy in the case of obligatory Hajj, and the expense exceeding the cost of al‑Hijjat al‑miqatiyyah istaken from the one third. (al‑Jawahir)
 
Delay by the Na'ib
Once the na'ib is hired, it is obligatory for him to act with immediacy. He may not postpone the Hajj beyond the first year. Also, it is not permissible for him to depute another, since the duty is his own. If we do not know that he actually went on the pilgrimage and performed all its essential acts, or if we doubt whether he performed them correctly and properly or not, or whether he failed to fulfil any of its obligatory essentials, then we assume that he acted correctly and properly, unless there is proof to the contrary.
 
Change of Purpose by the Na'ib (al‑Udal)
According to the Hanafi and Imamiyyah schools, if one species to the na'ib a particular kind of Hajj; such as Jajj al‑'ifrad, or Hajj al-qiran; then it is not permissible for him to make any change. However, if a particular town was specified as the starting point and the na'ib starts from another town, the purpose of the one who hires him is considered as fulfilled if the said specification was not really intended by the hirer; i.e. if by mentioning the route he meant the Hajj itself, and not the route specifically. (al‑Tadhkirah, al‑Fiqh ala al‑madhahib al‑'arbaah).
  Notes:

[13]. The Imamiyyah, Shafi'i, and Maliki schools permit hiring another person to perform the Hajj for a fee. The Hanafi and Hanbali schools do not consider it permissible. Nothing more than the expenses of journey, food and lodging may be given to the hired, they say.
[14]. The Qur'an, 22:78.
[15]. One who has not performed the Hajj before is called sarurah. According to the Shafi'i and Hanbali schools, if one who has not performed the Hajj before, undertakes it on behalf of another, the Hajj performed is considered his own. But according to the Maliki, Hanafi, and Imamiyyah schools, the Hajj performed depends on his intention (niyyah).
[16]. The minimum distance required for qasr in zuhr, 'asr and 'isha' prayers is 8 parasangs (approximately 44 kms. or 27.5 miles). (Tr.)