The authority for Amal of Ahl al-Madinah mainly derives from the following evidence:
Qadi ‘Iyad mentions in his Masalik: “It is related that ‘Umar ibn al-Khattab, may Allah be pleased with him, once said on the mimbar, ‘By Allah, I will make things difficult for anyone who relates a hadith which is contrary to the ‘amal of Madinah.
Malik and the Madhhab of Madina, Routledge, London, 2007, p.69.