Does the consensus of the people of Madinah constitute shar‘i proof?

Imam Maalik (may Allah have mercy on
him) was highly focused on the actions of the people of Madinah and
regarded that as proof in the religion of Allah and that it was not
permissible to go against their jamaa‘ah. He wrote to al-Layth ibn Sa‘d
(may Allah have mercy on him) saying: 

“The people should follow the example of
the people of Madinah. The Hijrah (migration of the Prophet (blessings and
peace of Allah be upon him)) was to Madinah; in that city Qur’an was
revealed, what is halaal was declared to be halaal and what is haraam was
declared to be haraam. The Messenger of Allah (blessings and peace of Allah
be upon him) lived among them and they were present when Revelation came
down. He commanded them and they obeyed him; he showed them the way and they
followed him, until Allah took his soul and chose for him that which is with
Him. May peace and blessings and mercy of Allah be upon him. 

Then after him came those among his ummah
who most closely followed his way of those who were appointed to positions
of authority after him. Whatever knowledge they had of what was revealed to
him, they implemented it, and whatever they did not have knowledge of, they
asked about it and discussed it with others, then they based their
conclusion on the strongest opinion that they were able to reach and they
were very close to the time of the Prophet (sa). If someone differed with
them, or someone else had stronger and sounder evidence, they would give up
their opinion and follow the other. Then the Taabi‘een came after them and
followed in their footsteps. 

If a certain practice was widely followed
in Madinah, I [Imam Maalik] do not think anyone has the right to differ from
it, because of what they have inherited and their unique situation that no
one else can claim to have. If any other city claimed that this practice
originated in their city, and that they grew up with it and it was the
practice of their predecessors, there could be no certainty that this
practice is sound and they cannot make the same claims as the people of
Madinah. 

End quote from Tarteeb al-Madaarik
by al-Qaadi ‘Iyaad, 1/10 

Secondly: 

What Imam Maalik (may Allah have mercy on
him) was referring to was the practice of the people of Madinah that they
handed down from one generation to another, and their old practices from the
time of the Sahaabah (may Allah be pleased with him), that were followed by
the people who came after them. As for their practices after that time and
their differences with the people of other cities with regard to issues that
could only be worked out on the basis of ijtihaad, Imam Maalik did not
regard that as evidence that it is not permissible to differ from; rather at
best it might support an argument. However, the later Maalikis differed from
him on this matter. 

Al-Qaadi ‘Abd al-Wahhaab ibn ‘Ali al-Maaliki
(may Allah have mercy on him) said: The consensus of the people of Madinah
based on what they narrated from the Prophet (blessings and peace of Allah
be upon him) constitutes proof that it is haraam to go against. As for their
consensus on matters that were worked out by means of ijtihaad, there is a
difference of scholarly opinion as to whether it constitutes evidence. The
correct opinion in our view is that we may regard it as more likely to be
correct than other views, but it is not haraam to differ from it.

End quote from al-Ma‘oona ‘ala Madhhab
‘Aalim al-Madinah, 2/607 

Al-Baaji (may Allah have mercy on him)
said: 

That is because Maalik relied on the views
and practices of the people of Madinah, and regarded them as proof if these
practices were based on narrations, such as the issue of the adhaan, not
reciting the phrase “Bismillah ir-Rahmaan ir-Raheem” out loud in the prayer,
the issue of the saa‘ (a unit of measurement), not giving zakaah on
vegetables, and other issues that were learned from narrations and reports
and were widely practised in Madinah in a manner that could not be hidden,
and was transmitted in such a way that it became binding proof and left no
excuse.

End quote from Ihkaam al-Fusool,
1/468 

He also said (1/488): 

With regard to what they reached by means
of ijtihaad and working it out, there is no difference between the views of
the scholars of Madinah and the views of other scholars, that the strongest
view is that which is based on the strongest evidence. Hence Maalik differed
with the people of Madinah concerning a number of issues. 

This is the view of Maalik concerning this
matter, and it is also the view of some of our companions, such as Abu Bakr
al-Abhari and others. It is also the view of Abu Bakr, Ibn al-Qassaar and
Abu Tamaam, and it is the correct view. 

A number of those who followed the Maaliki
madhhab but did not examine this issue fully thought that the consensus of
the people of Madinah constituted proof even in issues that are based on
ijtihaad. End quote. 

Shaykh al-Islam (may Allah have mercy on
him) said: 

The practice of the people of Madinah that
was based on narrations constitutes proof according to the consensus of the
Muslims, as Maalik said to Abu Yoosuf, when he asked him about the saa‘ and
the mudd (units of measurement), and Maalik instructed the people of Madinah
to bring out their saa‘s, and they told him that they had learned this from
their predecessors. Maalik said: Do you think they are lying, O Abu Yoosuf?
He said: No, by Allah they are not lying. 

End quote from Majmoo‘ al-Fataawa,
20/306 

And Allah knows best.