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

                I have heard that Imam Maalik used to regard the consensus of the people of Madinah as binding proof. Is this correct? If that is the case, then is it regarded as binding proof for the entire ummah?
Praise be to Allah.
      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.