Parashas Bamidbar · Passuk 4:18
Do not let the group of Kohathite clans be cut off from the Levites.
Chabad.org — English translation + Rashi
Source: Arachin 11b.
English: “Do not cut off” — it was taught: One who dies at fifty years — this is the death of kares.
In context: The warning against the Kehasites being cut off is linked to the age at which premature death is classified as kares.
Custom
Chazal associate the language of “cutting off” with the classification of death at fifty as kares.