Parashas Bamidbar · Passuk 4:12
They shall take all the service vessels with which the service in the sanctuary is performed, put them into a blue cloth and cover them with a covering of dolphin skin, which they shall then place on a carrying frame.
Chabad.org — English translation + Rashi
Source: Shabbos 98b.
English: “With which they serve” — a teaching of the school of R’ Yishmael: two vessels and one service; this teaches that if one placed a basin inside a basin and received the blood in them, it is valid.
In context: The plural language of service vessels yields a practical rule about nested utensils in the blood-reception ritual.
Custom
Chazal derive from the plural that nested basins may validly receive the blood.