Parashas Naso · Passuk 6:03
they shall abstain from wine and any other intoxicant; they shall not drink vinegar of wine or of any other intoxicant, neither shall they drink anything in which grapes have been steeped, nor eat grapes fresh or dried.
Chabad.org — English translation + Rashi
Source: Nazir 4a.
English: “From wine and strong drink” — “shechar” is said here and “shechar” is said elsewhere; just as there it is a revi’is, so here it is a revi’is.
In context: The gezerah shavah fixes the minimum quantity that constitutes a violation of the nazir’s prohibition.
Custom
Chazal establish the measure of a revi’is as the threshold for the nazir’s wine prohibition.