Allah (subhanahu wa ta’aala) says:
"But no, by your Lord, they will not have Eeman until they make you (O Prophet) rule between them in that which they dispute, and they find in their souls no resistance against your decisions, but accept them with the fullest conviction" (TMQ 4:65).
"Indeed, we have revealed to you the book with the truth so that you may rule between mankind by that which Allah has shown you" (TMQ 4:105).
"So rule between them by that which Allah has revealed, and follow not their desires, but beware of them in case they seduce you from just some part of that which Allah has revealed to you" (TMQ 4:49).
فَأُوْلَـئِكَ هُمُ الظَّـلِمُونَ
الْمُقْسِطِينَ فَأُوْلَـئِكَ هُمُ
"Whosoever does not rule by that which Allah has revealed, they are disbelievers (Kafiroon).....the thaalimoon (oppressors)....the fasiqoon (evil doers)" (TMQ 4. 5:44-47)
These ayaat (verses) of Qur'an, and many others, prove beyond doubt the obligation of ruling by what Allah has revealed. The first one in particular refers to the Muslims directly by stating that we have no real Imaan (belief) until we make them judge between us by Allah's revelation. This is an indication of the obligation for all Muslims to establish Allah's ruling system.
Our beloved Prophet (salAllahu alaihi wasallam) conveyed many sayings which emphasised the importance of the khilafah.
Imam Muslim narrated from Abu Hazim who said:
I was with Abu Hurairah for five years and I heard him narrate from the Prophet (salAllahu alaihi wasallam) that he said: "The Prophets used to rule Bani Israel. Whenever a prophet died another prophet succeeded him, but there will be no prophets after me; instead there will be Khulafaa' (Khalifahs) and they will number many". They asked: what then do you order us? He said: "fulfill allegiance to them one after the other. Give them their dues. Verily Allah will ask them about what he entrusted them with".
This Hadeeth is a clear statement of the fact that the form of government in Islam, after the Prophet (salAllahu alaihi wasallam) is the Khilafah, and not an Islamic Republic, Islamic Socialist Republic or Islamic Imarah. This understanding is supported by numerous other Ahadith that indicate the only system of government in Islam is the Khilafah.
Imam Muslim narrated from Abdullah bin 'Umar who said that the Prophet (saw) said,
"One who dies without having bound himself by an oath of allegiance (to a Khaleefah) will die the death of one belonging to the days of ignorance (Jahiliyah)".
Ahmed and Ibn abi 'Asim narrated that the Prophet (saw) said,
"Whosoever dies and he does not have over him an Imaam, he dies the death of Jahilyyah".
Thus the Prophet (salAllahu alaihi wasallam) made it compulsory that every Muslim should have over him an Imam, which is also represented by having a pledge of allegiance (bay’ah) on his or her neck. The pledge of allegiance is not given to anyone except the Khaleefah. The Ahadith inform us that those who run the affairs of Muslims are Khulafaa’a (some times called Amir ul-Mu'mineen or the Imam). Therefore, this is a command to establish or appoint them.
The classical scholars have clearly clarified the necessity of the Khilafah, despite the fact they live whilst it existed.
Sheikh Abd al-Rahman al-Jaziri (1299-1360 AH)
The Imams (scholars of the four schools of thought)- may Allah have mercy on them- agree that the Caliphate is an obligation, and that the Muslims must appoint a leader who would implement the injunctions of the religion, and give the oppressed justice against the oppressors. It is forbidden for Muslims to have two leaders in the world whether in agreement or discord. [Al-Fiqh Alal-Mathahib Al- Arba’a (the fiqh of the four schools of thought), volume 5, page 416]
Muhammad ibn Ali Al-Shawkani (d. 1250 AH)
It is known from Islam by necessity (bi-dharoorah - i.e.: like prayer and fasting) that Islam has forbidden division amongst Muslims and the segregation of their land. [Tafseer al-Quran al-Atheem, volume 2, page 215]
Imam Yahya ibn Sharaf al- Nawawi (631–676 AH)
(The scholars) consented that it is an obligation upon the Muslims to select a Khaleefah [Sharhu Sahih Muslim page 205 vol 12]
He also said:
It is forbidden to give an oath to two Imams or more, even in different parts of the world and even if they are far apart ["Mughni Al-Muhtaj", volume 4, page 132]
He also stated:
If a baya’a were taken for two Khulafa’aa one after the other, the baya’a of the first one would be valid and it should be fulfilled and honoured whereas the baya’a of the second would be invalid, and it would be forbidden to honour it. This is the right opinion which the majority of scholars follow, and they agree that it would be forbidden to appoint two Khulafa’aa at one given time, no matter how great and extended the Islamic lands become. ["Sharhu Sahih Muslim" (explanation of Sahih Muslim) chapter 12 page 231]
Imam Abu Abdullah al-Qurtubi (d. 671 AH)
Imam Qurtubi said in his tafseer of the verse,
This Ayah is a source in the selection of an Imaam, and a Khaleefah, he is listened to and he is obeyed, for the word is united through him, and the Ahkam (laws) of the Khaleefah are implemented through him, and there is no difference regarding the obligation of that between the Ummah, nor between the Imams
Imam al-Qurtubi also said: The Khilafah is the pillar upon which other pillars rest
Hujjatul Islam Imam Abu Hamid Al-Ghazzali
Imam Al-Ghazzali (450-505 AH), when writing of the potential consequences of losing the Khilafah said:
The judges will be suspeneded, the Wilayaat (provinces) will be nullified, ... the decrees of those in authority will not be executed and all the people will be on the verge of Haraam. [al Iqtisaad fil Itiqaad page 240]
Imam Taqi ad-Din Ahmad Ibn Taymiyyah (661-728 AH)
It is obligatory to know that the office in charge of commanding over the people (ie: the post of the Khaleefah) is one of the greatest obligations of the Deen. In fact, there is no establishment of the Deen except by it....this is the opinion of the salaf, such as al-Fadl ibn 'Iyaad, Ahmed ibn Hanbal and others. [Siyaasah Shariyyah - chapter: 'The obligation of adherence to the leadership']
Qadi Abu al-Hasan Ali Ibn Muhammad Ibn Habib al-Mawardi (362-448)
Imamate is prescribed to succeed prophethoood as a means of protecting the Deen and of managing the affairs of this world. There is a consensus of opinion (amongst the scholars - Ijmaa’ al-‘Ulamaa’) that the person who discharges the responsibilities of this position must take on the contract of Imamate of the Ummah. [Al-Ahkam us-Sultaniyyah, Chapter 1, The Contract of Imamate].
Imam Ahmad ibn Hanbal (164-241 AH)
The Fitna (mischief and tribulations) occurs when there is no Imaam established over the affairs of the people.
Abu Hafs Umar al-Nasafi (d. 701)
The Muslims simply must have an Imam (Khaleefah), who will execute the rules, establish the Hudud (penal system), defend the frontiers, equip the armies, collect Zakah, punish those who rebel (against the state) and those who spy and highwaymen, establish Jum'ah and the two 'Eids, settle the dispute among the servants (of Allah), accept the testimony of witnesses in matters of legal rights, give in marriage the young and the poor who have no family, and distribute the booty.
Imam Al-Haythami (d. 807 AH)
It is known that the Sahabah (ra) consented that selecting the Imaam after the end of the era of Prophethood was an obligation (Wajib). Indeed they made it (more) important than the (other) obligations whilst they were busy with it over the burial of the Prophet. [Sawaa'iq ul-haraqah:17]
Imam Sa'd al-Din Mas'ud bin Umar al-Taftazani (d. 791 AH), wrote:
There is consensus that appointing a Khaleefah is obligatory. The difference of opinion is on whether the appointment must be by Allah or by his servants, and whether the basis (for appointment) is textual evidence or rational proof. The adoption is that it is obligatory upon the servants by textual evidence because of the saying of the Messenger, "Whoever dies not having known the Imam of his time, dies the death of the days of ignorance." Also, the ummah agreed that this was the most important duty following the death of the Messenger, so important in fact that they considered it more important than the matter of his burial, and so also has it been after the death of each Imam.