TT #238 · Parshas Shemini · Aliyah 7 · Passuk 41
Chabad.org — Vayikra Chapter 11 with Rashi
English: “The creeping thing that creeps”: Rav Huna cautions against pouring beer into a vessel at night, lest a creature separate above the vessel and fall back into the cup, causing the prohibition to be violated.
Chullin 67a.
E-Daf.com — Chullin 67a (Tsurras Hadaf)
Practical application — The Chazal applies the verse’s wording to establish the stated rule.
The source describes a practical case where a sheretz can enter a drink.