Hadith: Anyone pledges allegiance to a ruler, giving him the grasp of his hand and sincerity of his heart, he should obey him to the best of his capacity. If another man comes forward disputing his authority, then you should behead the latter

عن عبد الله بن عمرو رضي الله عنهما قال: كنا مع رسول الله - صلى الله عليه وسلم- في سفر، فَنَزَلنَا مَنْزِلًا، فَمِنَّا مَنْ يُصلِح خِبَاءَه، ومِنَّا من يَنتَضِل، ومِنَّا مَن هو في جَشَرِهِ، إِذْ نادى مُنادي رسول الله صلى الله عليه وسلم : الصَّلاةُ جَامِعَةٌ. فاجْتَمَعنَا إلى رسول الله - صلى الله عليه وسلم - فقال: «إِنَّه لَمْ يَكُن نبي قبْلِي إِلاَّ كَان حَقًّا عليه أَنْ يَدُلَّ أُمَّتَه عَلَى خَيرِ مَا يَعْلَمُهُ لَهُم، ويُنذِرَهُم شّرَّ ما يعلمه لهم، وإِنَّ أُمَّتُكُم هذه جَعَل عَافِيَتَهَا في أوَّلِها، وَسَيُصِيبُ آخِرَهَا بَلاَءٌ وأُمُورٌ تُنكِرُونَهَا، وتَجِيءُ فِتنَةٌ يُرَقِّقُ بَعْضُهَا بَعضًا، وتَجِيءُ الفتنة فيقول المؤمن: هذه مُهلِكَتِي، ثُمَّ تَنْكَشِف، وتجيء الفتنة فيقول المؤمن: هَذِه هذِه. فمَنْ أَحَبَّ أنْ يُزَحْزَحَ عن النار، ويدخل الجنة، فَلْتَأْتِه مَنِيَتُهُ وهو يؤمن بالله واليوم الآخر، وَلْيَأتِ إِلى النَّاسِ الَّذِي يُحِبُّ أَنْ يُؤْتَى إليه، ومَنْ بَايَع إِمَامًا فَأَعْطَاه صَفْقَةَ يَدِهِ، وَثَمْرَةَ قلْبِهِ، فَلْيُطِعُه إِن اسْتَطَاع، فَإِن جَاء آخَرُ يُنَازِعُه فَاضْرِبُوا عُنُقَ الآخَرِ».
[صحيح] - [رواه مسلم]

Abdullāh ibn ‘Amr (may Allah be pleased with him) reported: We were on a journey with the Messenger of Allah (may Allah's peace and blessings be upon him) and we halted at some place to rest. Some of us began to mend their tents; others began to compete with one another in shooting; and others began to graze their beasts. An announcer of the Messenger of Allah (may Allah's peace and blessings be upon him) announced that we should gather for prayer. So we gathered around the Messenger of Allah (may Allah's peace and blessings be upon him). He said: "It was the duty of every prophet before me to guide his people to what he knew was good for them and warn them against what he knew was bad for them. The safety of this Ummah of yours lies in its early generations, whereas its late generations will be afflicted with trials and with things that you would reject. There will be tremendous trials, one after the other, each making the previous one dwindle into significance. When they are afflicted with a trial, the believer will say: 'This is going to bring about my destruction.' When it is over, they will be afflicted with another trial and the believer will say: 'This surely is going to be my end.' Anyone wishes to be saved from the Hellfire and be admitted to Paradise should die while having belief in Allah and the Last Day, and should treat people as he wishes to be treated by them. Anyone pledges allegiance to a ruler, giving him the grasp of his hand and sincerity of his heart, he should obey him to the best of his capacity. If another man comes forward disputing his authority, then you should behead the latter."
[Authentic hadith] - [Narrated by Muslim]

Explanation

This Hadīth states that it is the duty of scholars, just as it was the duty of the prophets, to clarify all forms of good, encourage people to perform it, and guide them to its path. It is also their duty to clarify all forms of evil and warn people against it. The Hadīth shows that the early generations of this Ummah were granted safety from trials, whereas its late generations will be afflicted with evil and trials that will make the coming trials trivialize the previous trials. Safety (from these trials) lies in monotheism, adherence to the Sunnah, kind treatment of people, abidance by the pledge given to the ruler and refraining from rebelling against him, and killing whoever wishes to create dissension among the Muslims.