Evidence of this principle can be looked at from two sides:
Firstly: The Shari'ah has given the right of appointing the Khaleefah to
the Ummah; in other words it is the Ummah who chooses the Khaleefah and
gives him the pledge of allegiance. This was accomplished by the pledge of
allegiance given to him by the Muslims. This is backed by many ahadith
from which we list the following:
Ubada ibn al-Samit reported, "We pledged ourselves in complete obedience
to the Messenger of Allah in wealth and woe..."
Jarir ibn 'Abdullah reported, "I pledge myself in complete obedience to
the Messenger of Allah."
Abu Hurayrah (ra) reported, "The Messenger of Allah (saw) said: Three
types of people Allah will not talk to on the Day of Reckoning, nor would
he forgive them; they would be severely punished: A man who has water to
spare and would not give it to the traveller, a man who gives his pledge
to an Imam but not for his deen - he would only obey him if he gave him
what he wanted, otherwise he would not, and a man who would strike a deal
with another man after 'Asr, swearing by Allah that he was given so much
for the goods and he was not given so."
Secondly: The Shari'ah allows the Khaleefah to take the authority from the
Ummah once it gives him the pledge of allegiance then the Ummah is obliged
to obey him for he is a Khaleefah with a bay'ah (pledge). So the authority
is given to the Khaleefah by the Ummah by giving the pledge of allegiance
(bay'ah al-ta'aa) to obey him. This indicates that the authority is to the
Ummah.
Evidence about the Khaleefah taking the authority by a pledge can be
deduced from the following: 'Abdullah ibn 'Amr ibn al-'A'as reported that
he heard the Messenger of Allah (saw) saying, "Whosoever pledges
allegiance to an Imam giving him the clasp of his hand and the fruit of
his heart shall obey him as long as he can, and if another comes to
dispute with him strike the neck of that man." (Muslim)
Nafi'a reported that 'Abdullah ibn 'Umar told him that he heard Allah's
Messenger (saw) saying, "Whosoever takes his hand from allegiance to Allah
will meet him on the Day of Resurrection without any evidence supporting
him and whosoever dies while there was no allegiance on his neck dies a
death of the days of ignorance." (Muslim)
Many other ahadith indicate that the authority is to the Ummah for it
chooses a man from among it, gives him the authority and gives him the
pledge of allegiance according to the Book of Allah and the Sunnah of His
Messenger (saw).