Parashas Naso
וְנָ֨שְׂא֜וּ אֶת־יְרִיעֹ֤ת הַמִּשְׁכָּן֙ וְאֶת־אֹ֣הֶל מוֹעֵ֔ד מִכְסֵ֕הוּ וּמִכְסֵ֛ה הַתַּ֥חַשׁ אֲשֶׁר־עָלָ֖יו מִלְמָ֑עְלָה וְאֶ֨ת־מָסַ֔ךְ פֶּ֖תַח אֹ֥הֶל מוֹעֵֽד׃
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;
Torah Temimah points to TT / Ohalos related (verified).
מכסהו ומכסה התחש — מקיש מכסה עליון למכסה תחתון: מה תחתון קרוי אוהל לטומאה אף עליון קרוי אוהל לטומאה.
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.
Custom
Chazal derive that both the lower and upper coverings of the Mishkan function as an ohel for impurity.