Surah Al-Ma'arij 70:8

يَوْمَ تَكُونُ ٱلسَّمَآءُ كَٱلْمُهْلِ

On the Day the sky will be like murky oil,

Translation: Saheeh International

Tafsir

The day when the heaven will be yawma takūnu’l-samā’u is semantically connected to an omitted clause implicitly taken to be yaqa‘u ‘it will take place’ as molten silver

Excerpt from Tafsir al-Jalalayn, commentary on 70:8, via Quran.com. The reader has the full commentary and 9 more tafsirs.