What is the source of the practice not to shower after mikvah?

Among the famous eighteen decrees instituted by Beis Shammai and accepted as halacha is that one who immerses in a kosher mikvah and afterward has three lugin (around 1 liter) or more of drawn water (mayim sheuvim) poured over their head and most of their body becomes tamei once again.¹

Mikvaos in those times were often murky, so it was natural for people to rinse themselves off after immersion. Some began to mistakenly think it was the clean, drawn water that accomplished their tahara, and Chazal were concerned that the actual mikvah immersion would be abandoned altogether. To prevent this error, Chazal decreed that non-mikvah water poured on a person after immersion would itself bring back tumah, discouraging people from doing it .²

Rishonim disagree on the effect of this decree. Some understand that the rinsing completely undoes the original tevilah, and the person must immerse again;³ others hold that only a weaker form of tumah returns; and some limit the entire decree to the era when people ate terumah, and not to tevilah in our time.⁴ While the Mechaber doesn’t mention it,⁵ the Rama rules to be machmir with tevilah de’oraisa.⁶

For tevilah before davening, or in honor of Shabbos and Yom Tov, some contemporary poskim hold that there is no halachic concern with showering afterwards⁷ (in fact, tevilas Ezra is itself valid even when done in mayim sheuvim⁸). Others nevertheless conclude that it is preferable not to shower after mikvah, especially when immersing for tosefes tahara.⁹

Chassidim were particular not to wash off the waters of the mikvah even when those waters were very dirty, and even on a regular weekday, due to the holiness of the mikvah waters.¹⁰

The Arizal teaches that the erev Shabbos mikvah waters should not be dried from the body at all, so the body can soak in kedusha.¹¹ In practice, the Rebbe said that one can fulfill this by not drying one part of the body.¹²

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