Parashas Bamidbar · Passuk 4:15
bContinuing v. 4. When Aaron and his sons have finished covering the sacred objects and all the furnishings of the sacred objects at the breaking of camp, only then shall the Kohathites come and lift them, so that they do not come in contact with the sacred objects and die. These things in the Tent of Meeting shall be the porterage of the Kohathites.
Chabad.org — English translation + Rashi
Source: Sanhedrin 81b.
English: “The burden of the sons of Kehas” — it was taught: R’ Elazar said: One who carries a burden on Shabbos higher than ten handbreadths is liable.
In context: The manner in which the Kehasites carried the sacred objects on their shoulders supplies a model for the melachah of hotza’ah.
Custom
Chazal use the Kehasite carrying method as a paradigm for liability in carrying on Shabbos.