Musar על חגיגה 32:22

Even matters of Shevut (Resting), despite that they are Rabbinical, are nevertheless important principles as the Sages said: "Never let [the principle] of Shevut (Rest) be light in your eyes. For the laying on of the hands [on a sacrifice for a Festival-day] is [prohibited] only on account of Shevut, yet the greatest men of the age debated therein" (Chagigah 16b).