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
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.