Ruling in Islam
Ruling (al-hukm), reign (al-mulk) and authority (al-sultan) have the
same meaning which is the authority that executes the rules, or
it is the task of the leadership (al-imarah) which the Shara has
made an obligation on Muslims to establish. The task of al-imarah is to
avert injustice and to settle disputes that may arise. In other words the
ruling means the guardianship, as revealed by Allah. He says:
Obey Allah and obey the Messenger and those of you in authority. [TMQ An-Nisa: 59]
And He also says:
If they had only referred it to the Messenger and those among them with authority.
[TMQ An-Nisa: 83]
Therefore to rule is to actively look after the peoples affairs.
Since Islam is an ideology that covers the state, society and life as a
whole, the ruling becomes part of it, and the Muslims are commanded
to implement this ruling by establishing the state i.e. to govern by the
Islamic laws. A host of verses have been revealed in the Quran
confirming the obligation of ruling by what Allah  has revealed.
Allah says:
So judge between them by that which Allah has revealed and follow not their
desires away from the truth that has come to you.
[TMQ Al-Maidah: 48]
He also says:
Judge between them by that which Allah has revealed and follow not their desires
and beware of them lest they seduce you from some part of that which Allah has
revealed to you. [TMQ Al-Maidah: 49]
Allah  says:
Whosoever does not judge by that which Allah has revealed, such are disbelievers.
[TMQ Al-Maidah: 44]
Whosoever does not judge by that which Allah has revealed, such are oppressors.
[TMQ Al-Maidah: 45]
Whosoever does not judge by that which Allah has revealed, such are transgressors.
[TMQ Al-Maidah: 47]
He  also says:
But no, by your Lord, they can have no (real) faith until they make you judge in all disputes between them and find in their souls no resistance against your decisions,
but accept them with the fullest submission. [TMQ 4:65]
He  also says:
Obey Allah and obey the Messenger and those of you in authority. [TMQ An-Nisa: 59]
He  also says:
And if you judge between people, judge with justice. [TMQ An-Nisa: 58]
This is in addition to many other verses dealing with ruling as an
authority and power. Other verses have also been revealed providing
details with respect to the different areas of ruling; some regarding
military legislation, political legislation, criminal legislation, social
legislation and the legislation of transactions as well as others. Allah 
says:
Fight those of the disbelievers who are near you and let them find harshness in you.
[TMQ At-Tauba: 123]
He  also says:
If you come on them in the war, deal with them so as to strike fear in those who So judge between them by that which Allah has revealed and follow not their
desires away from the truth that has come to you.
[TMQ Al-Maidah: 48]
He  also says:
Judge between them by that which Allah has revealed and follow not their desires
and beware of them lest they seduce you from some part of that which Allah has
revealed to you. [TMQ Al-Maidah: 49]
Allah  says:
Whosoever does not judge by that which Allah has revealed, such are disbelievers.
[TMQ Al-Maidah: 44]
Whosoever does not judge by that which Allah has revealed, such are oppressors.
[TMQ Al-Maidah: 45]
Whosoever does not judge by that which Allah has revealed, such are transgressors.
[TMQ Al-Maidah: 47]
He  also says:
But no, by your Lord, they can have no (real) faith until they make you judge in all disputes between them and find in their souls no resistance against your decisions,
but accept them with the fullest submission. [TMQ 4:65]
He  also says:
Obey Allah and obey the Messenger and those of you in authority. [TMQ An-Nisa: 59]
He  also says:
And if you judge between people, judge with justice. [TMQ An-Nisa: 58]
This is in addition to many other verses dealing with ruling as an
authority and power. Other verses have also been revealed providing
details with respect to the different areas of ruling; some regarding
military legislation, political legislation, criminal legislation, social
legislation and the legislation of transactions as well as others. Allah 
says:
Fight those of the disbelievers who are near you and let them find harshness in you.
[TMQ At-Tauba: 123]
He  also says:
If you come on them in the war, deal with them so as to strike fear in those who are behind them, that they may remember. If you fear treachery from any group throw
back (their covenant) to them (so as to be) on equal terms. [TMQ Al-Anfal: 57-58]
He  also says:
And if they incline to peace, incline you also to it, and trust in Allah. [TMQ Al-Anfal: 61]
He  also says:
O you who believe fulfil the contracts (undertakings).
[TMQ Al-Maidah: 1]
And eat up not your property among yourselves in vanity, nor seek by it to gain the
hearing of the judges that you may knowingly devour a portion of the property of the
others wrongfully. [TMQ Al-Baqarah: 188]
And there is (a saving of life) life for you in (the law of) retaliation, O men of
understanding. [TMQ Al-Baqarah: 179]
As for the thief, both male and female, cut off their hands, as a retribution for their
deed and an exemplary punishment from Allah.
[TMQ Al-Maidah: 38]
Then, if they suckle for you, give them their due payment.
[TMQ 65:6]
Let him who has abundance spend of his abundance, and he whose provision is
measured, let him spend according to what Allah has given him. [TMQ At-Talaq:7]
Take alms of their wealth, with which you may purify them.
[TMQ At-Tauba:103]
Thus, we find the broad lines of the legislation in its various aspect,
including the military, criminal, political and transactions evident in
hundreds of verses in addition to the numerous sound Ahadith of the
Messenger of Allah , which have all been revealed to be implemented
and executed. They have effectively been implemented in the lifetime of
the Messenger of Allah , the period of the Khulafaa Ar-Rashidoon, as
well as during the ruling of the Khulafaa who came after them. This
could only prove that Islam is a system for ruling and state, for society
and life, and for the Ummah and the individuals. It also proves that the
state has no authority for ruling unless it adheres to the Islamic system.
Islam would have no existence unless it was alive, represented by a state
that implements its rules. Islam is a deen (religion) and an ideology, with
the State and the ruling system being part of it. The State is the only
legal body that Islam has laid down to implement its laws and execute
them in public life. Islam would not exist without a state that implements
it under all conditions. Furthermore, the State is a political and human
entity, not a holy or spiritual one. It has no sainthood and its leader has
no infallible characteristics.
The ruling system is the one that determines the shape of the State and
its character, its foundation, pillars and bodies, and the basis upon which
it is built, together with the basic thoughts, concepts and the criterion
with which it looks after the affairs, as well as the constitution and the
laws it implements.
It is a distinct and unique system for a distinct and unique state, being
totally different from all other systems known to mankind whether in
terms of the basis upon which it is built or the thoughts, concepts and
criteria with which it deals with peoples affairs, or the shape that it takes
or the laws and constitution it implements.