Parashas Bamidbar
וְלָקְחוּ֩ אֶת־כׇּל־כְּלֵ֨י הַשָּׁרֵ֜ת אֲשֶׁ֧ר יְשָֽׁרְתוּ־בָ֣ם בַּקֹּ֗דֶשׁ וְנָֽתְנוּ֙ אֶל־בֶּ֣גֶד תְּכֵ֔לֶת וְכִסּ֣וּ אוֹתָ֔ם בְּמִכְסֵ֖ה ע֣וֹר תָּ֑חַשׁ וְנָתְנ֖וּ עַל־הַמּֽוֹט׃
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.
Torah Temimah points to Tanna de-vei R’ Yishmael — TT (verified).
אשר ישרתו בם — תנא דבי ר׳ ישמעאל: שני כלים ושירות אחת, מלמד שאם הניח מזרק בתוך מזרק וקבל בהן את הדם כשר.
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.
Custom
Chazal derive from the plural that nested basins may validly receive the blood.