Parashas Naso · Passuk 4:25
they shall carry the cloths of the Tabernacle, the Tent of Meeting with its covering, the covering of dolphinddolphin See note at v. 6. skin that is on top of it, and the screen for the entrance of the Tent of Meeting;
Chabad.org — English translation + Rashi
Source: Shabbos 98b.
English: “Its covering and the covering of tachash” — Scripture compares the upper covering to the lower: just as the lower is called a tent for impurity, so the upper is called a tent for impurity.
In context: The hekesh establishes that both layers of covering create an ohel for the laws of tumah.
Custom
Chazal derive that both the lower and upper coverings of the Mishkan function as an ohel for impurity.