Obey those in authority:
النساء
إِنَّ اللَّهَ يَأْمُرُكُمْ أَنْ تُؤَدُّوا الْأَمَانَاتِ إِلَى أَهْلِهَا وَإِذَا حَكَمْتُمْ بَيْنَ النَّاسِ أَنْ تَحْكُمُوا بِالْعَدْلِ إِنَّ اللَّهَ نِعِمَّا يَعِظُكُمْ بِهِ إِنَّ اللَّهَ كَانَ سَمِيعًا بَصِيرًا(58)يَاأَيُّهَا الَّذِينَ ءَامَنُوا أَطِيعُوا اللَّهَ وَأَطِيعُوا الرَّسُولَ وَأُولِي الْأَمْرِ مِنْكُمْ فَإِنْ تَنَازَعْتُمْ فِي شَيْءٍ فَرُدُّوهُ إِلَى اللَّهِ وَالرَّسُولِ إِنْ كُنْتُمْ تُؤْمِنُونَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ ذَلِكَ خَيْرٌ وَأَحْسَنُ تَأْوِيلًا(59)
Final authority over the Ummah is to only one person. There can never be more than one or less than one.
مسلم
و حَدَّثَنِي وَهْبُ بْنُ بَقِيَّةَ الْوَاسِطِيُّ حَدَّثَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ عَنْ الْجُرَيْرِيِّ عَنْ أَبِي نَضْرَةَ عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا بُويِعَ لِخَلِيفَتَيْنِ فَاقْتُلُوا الْآخَرَ مِنْهُمَا
We believe that the supreme imamship is one of the most critical purposes of Islam and one of its clearest obligations. It occupies the position of Allah's Messenger (sas) after him in safeguarding this religion on Allah's earth and directing the affairs of this world. Being a fardh (obligation) upon the Ummah as a whole, the Muslims do not achieve innocence of sin until they unite under a single Imam on this earth who directs their affairs according to Allah's Book and the Sunnah of His Messenger (sas).
The verse from An-Nisaa cited earlier orders us to "...render all trusts unto the appropriate parties and when you judge between people, judge with justice...". The address here is general, to all the Muslims, that they must render and fulfill all sorts of trusts. One of the most important trusts is that of governing. And so it obligatory upon the Ummah to seek the means of entrusting that very important function to one who is appropriate to it and who will fulfill it as it should be fulfilled.
This principle of imama ("imamship") or amara ("amirship" or "rulership") is at all levels in Islam. it is the means of social organization which Muslims must use in all kinds of endeavors. The Prophet (sas) said:
أحمد
حَدَّثَنَا حَسَنٌ حَدَّثَنَا ابْنُ لَهِيعَةَ قَالَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ هُبَيْرَةَ عَنْ أَبِي سَالِمٍ الْجَيْشَانِيِّ عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ لَا يَحِلُّ أَنْ يَنْكِحَ الْمَرْأَةَ بِطَلَاقِ أُخْرَى وَلَا يَحِلُّ لِرَجُلٍ أَنْ يَبِيعَ عَلَى بَيْعِ صَاحِبِهِ حَتَّى يَذَرَهُ وَلَا يَحِلُّ لِثَلَاثَةِ نَفَرٍ يَكُونُونَ بِأَرْضِ فَلَاةٍ إِلَّا أَمَّرُوا عَلَيْهِمْ أَحَدَهُمْ وَلَا يَحِلُّ لِثَلَاثَةِ نَفَرٍ يَكُونُونَ بِأَرْضِ فَلَاةٍ يَتَنَاجَى اثْنَانِ دُونَ صَاحِبِهِمَا
Aside from this, another absolute proof of the obligation of imamship is the complete ijmaa' (legal concensus) of the Muslims on this issue. The Companions of Allah's Mesenger were unanimous regarding the imamship right after the passing of Allah's Messenger (sas). So much so that they involved themselves with fulfilling this obligation immediately - even before preparing and burying the body of Allah's Messenger! The imams and scholars of this Ummah remained unamimous on this issue throughout the centuries.
Another equally clear although somewhat less direct proof of the obligation of imamship is the existence of many obligatory issues in Islam which cannot be fulfilled without a khalifa ruling over the Muslims. Examples include enacting the hudood and many other rulings of the Shari'a, defending Muslim lands, maintaining an army, assigning judges, etc. The principle in fiqh is "That without which an obligation cannot be fulfilled is itself obligatory." All of thos shows clearly that a single imam for the Ummah who manages their affairs according to the Shari'a. This is a clear obligation of Islam and there is no way it can be legitimately neglected or that its obligation can be disputed or questioned.
Ali (ra) said:
لا بد للناس من امارة برة كانت او فاجرة. قالوا: يا أمير المؤمنين هذه البرة عرفناها , فما بال الفاجرة؟ قال: تقام بها الحدود وتأمن بها السبل, ويخاهد بها العدو وتقسم بها الفئ