Babylonian Talmud, Baba Metzia 83a

We need [this example in the Mishnah specifying that local custom undermines an employer’s stipulation that workers begin early and stay late] for the case in which the employer raises the workers’ wages. In the case in which the employer says to them, “I raised your wages in order that you would begin work early and stay late,” they may reply, “You raised our wages in order that we would do better work.” [Soncino translation. Edited for gender neutrality]