It’s often said that when Hillel instituted the current calendar, he set an “expiration date” of 6000 AM; at that point the calendar would be invalid. Since Mashiach presumably will come by then (as per Sanhedrin 97a et. al.) the calendar doesn’t need to run farther than that.
As often as I’ve heard this repeated, I’ve yet to find a concrete source for it. @Shalom asks a question here regarding the calendar post-6000, and @DanielBilar asks here whether the existence of a month hangs on its sanctification and thus if we even can have a calendar post-6000, barring Mashiach’s imminent arrival. In discussing this point with others in the comments to the latter question, we arrived at the Rambam, Hilchos Kiddush HaChodesh 5:2ff who says nothing about sanctification, merely that we can use a calendar if there’s no Sanhedrin around to sanctify the month, and a reference to the Netziv and other poskim, but it doesn’t have any sources.
Does anyone know where I can find someone who says this explicitly, that the calendar’s existence hangs on the fact that Hillel and his Beis Din sanctified the calendar for the following 1600 years, only up to the year 6000 in anticipation of Mashiach’s arrival?