Bismillah,
No muslim disagrees to the fact that in all matters, it is the ruling
of Allah Almighty which has to be followed and obedience of His rules
is obligatory upon every muslim.Our acceptance of the rulings of
various Prophets and messengers (AS) , is only because they are the
carriers of the rulings of Allah and act as a connection between the
Creator and the created world.
If these messengers base some ruling on their own research or indepth
study (ijtihad), we accept it because they do so only after viewing it
from all angles and finding it in consensus with the rulings of
Almighty Allah and more so because Allah Almighty Himself has made it
obligatory upon the Creation to follow the messengers; otherwise "
innal hukma lillah" ( no ruling accept that of Allah Subhanahu wa
Ta'ala is acceptable) is the Bold and Clear declaration of the Quran .
It follows that just like following the rulings of the Messenger
cannot be said to be shirk in the authority of Allah swt, similarly
the Imams of the ummah who were the deputies of the Messenger and
spent all their energies and faculties in completely understanding in
depth the rulings of the Messenger; following the rulings deduced by
them, cannot constitute shirk fi risalah. The scholars or Imams are in
reality the propagators of the message of the Messenger; even the
rulings deduced by them after research and indepth study (ijtihad) are
actually rooted in the Quran and the Sunnah; so just like following
the messengers is in reality following Allah, similarly following the
scholars or the Imams is in reality following Allah and His messenger
(sallalahu allaihi wasalam).
The Quranic verse " Fas'aloo ahla zikri in kuntum la ta'alamoon"
appearing several times in the Quran teaches us the fundamental
concept that those people who cannot do an indepth study of the Quran
and the hadith should turn to those who know (the depths of the Quran
and sunnah) and then follow them.
Thus the basis of 'Taqleed' is only that those who do not know should
act on the rules of Allah only after asking 'how' , from those who
know. No intelligent human can deny the importance of this concept and
as such there is no controversy on this in the muslim ummah. The only
issue which is debated upon is whether following only one scholar in
all matters (taqleed e shakhsi) is permissible or not .If one
understands the basis of this matter unbiasedly, it is not too
difficult to understand it.

There are many verses in the Quran as well as a number of ahadith
which prohibit the total obedience of desires (itiba-e-hawa) ;
therefore a consensus exists among the muslim community as well as the
scholars that this 'itiba-e-hawa" is a haram act as per the rulings of
the shariah.
A person who acts according to his own will and desires and then looks
for the confirmation of such rulings in the Quran and ahadith , he
falls in the category of those who submit to their own will and desire
and cannot be said to be a follower of the Quran even though there
might exist some ruling in the Quran which is in accordance to what
his desire implies; keeping in mind the fact that this is occuring
between him and the One who is 'Aleem and Khabeer' and is fully aware
of the intention as well as the subtleties of our desires.
Hafiz ibn Taimiah (RA) writes in one of his fatawa that it is the
consensus of the entire ummah that a person who looks into the madahib
of the imams with the intention of finding an outlet for any of his
own desires and then follows what he desires while attributing his
action to any of the rulings of the Imams, cannot be said to be a
follower of Allah and His Rasool (sallalahu allaihi wasalam) but he is
only a follower of his own desire (hawa); doing this is turning the
entire deen into a mere toy and playing with it.
Ibn Taimia ( RA) says, " These people at times follow an imam who has
declared a particular kind of nikah as null and then at another time
follow another imam who declares such a nikah as right. This is just
following ones desires and according to the consensus of the entire
ummah is haram; (three lines after this ) he writes, " the similitude
of such a person is like the one who at one time desiring a way out of
his own nikah decides to follow imam Abu Hanifa and if somebody else
(known to him) wants the same ruling but this time his desire is that
the person should maintain his/her nikah , he decides to follow imam
shafi ! If the person says that initially I was unaware of the right
ruling but now that I know it I shall follow it , still his words are
not to be trusted as it is just like playing with the deen and becomes
a pretext for making ones own desire (hawa) as the touchstone for
declaring things halal or haram.

In short the obedience of hawa is by consensus a haram act and by
experience also one can state that if the general masses are left free
to decide rulings about various issues on their own , ie whenever they
wish they can follow imam Abu Hanifa and whenver they wish they can
follow imam Shafaee or any other imam, the result will be the same
situation which Hafiz Ibn Taimiya declares as 'haram'. Taking this
shara'ee consideration into view it was found safer to follow one imam
in all rulings .
The real goal is to protect oneself from itiba-e- hawa and since in
these times there is no other solution except to follow one imam so
this itaba of one imam takes the degree of wujoob or becomes wajib on
a muslim.
It follows that it is not essential that there exists a ruling in the
Quran and hadith making this following of an imam essential as the
Quran and Hadith exist to explain the rules of the shariah and do not
demarcate the means of attaining them; just like the Haj Pilgrimage is
obligatory (fardh) based on the Quran and Ahadith but none of these
two authentic sources delineate the means of transport one should
adopt for this pilgrimage.; if one goes on looking for the names of
trains, aeroplanes, ships etc in the books of Hadith and in the Quran
then it is a proof of one's sheer ignorance and lack of right
knowledge.. Thus even if there were no clear cut orders for following
an authentic imam in the Quran and hadith, the forbiddence of itiba-e-
hawa would have been enough to make it essential.
But looking into the lives of the blessed companions (sahaba ) (RA)
one can see that even though these stalwarts were free from the
dangers of itiba-e- hawa, still they would at times after proper
selection and at other times without even a prior selection follow a
scholar or theologist and then act according to his rulings; some of
the companions even at that time would follow a single scholar in all
matters and to prove this, the following authentic and famous
example should be enough:

In Sahih Bukhari , Hazrat Ikrimah (RA) narrates, " The people of
madinah once asked Hazrat Ibne Abbas (RA) regarding a lady who after
performing the obligatory (fardh) tawaf of the kaba got her menstrual
periods ; whether she should wait till her period ends and then
perform the tawaf-e- wida (the parting tawaf) or is it that she no
longer needs to perform the tawaf and can leave. Ibn Abbas (RA)
replied that she can leave. The questioners on hearing this reply
said, " we will not prefer your word over the ruling of Zaid bin
Thabit (RA) and will not act against his ruling".
In Fathul Bari ( the sharah of Sahih Bukhari), the following words
about the same incident narrated by Qatada (RA) are written," The
Ansar said ," we will not follow you and act against the ruling of
Zaid bin Thabit (RA)" . Ibn Abbas replied, " Go and ask umme Sulaim
about the issue (that the ruling I am giving you is the correct one
)".

Two facts get clarified by this conversation between the Ansar and
Ibne Abbas (RA) ; One - that the Ansar used to follow Zaid bin Thabit
(RA) in person (taqleed e shakhsi) and would never act against any of
his rulings. Second- that even Ibn e Abbas did not object to their
doing so as do the so called modern scholars of our times i.e go on
repeating that taqleed e shaksi is shirk fi risalah - it is haram and
not permissible .Hazrat Ibne Abbas (RA) only advised them to go back
to Zaid bin Thabit for confirmation of the ruling.
It is continued in Fathul Bari that these people then went to umme
Sulaim enquirng about the matter and then returned to Zaid bin Thabit
(RA) who referred back to the particular hadith and then accepted the
ruling of Hazrat Ibne Abbas (RA) as being correct and took back his
former fatwa.
In short what is clear from the incident is the consensus of the
Ansars and Hazrat Ibne Abbas on the ruling that those people who do
not possess the required depth of knowledge which qualifies one as a
mujtahid should turn to one among the scholars and make it a point to
follow him on all matters.
From this incident , the matter of taqleed e shakhsi proves to be
permissible for the people belonging to the times of the sahaba (RA) .
But in the times that followed, its becoming wajib (near to
obligatory) is because without it, it becomes impossible to safeguard
oneself from itibaa-e - hawa.
(Translation of " The proof for 'Taqleed e shaksi'-( Taqleed e shaksi
ka thaboot) Movlana Shabir Ahmad Sahab uthmani .