Tafsir of Al-Qamar 54:38

"And indeed, they were struck by a punishment in the morning."

The term bukratan refers to the beginning of the day; it is more specific than al-sabah (the morning), and mentioning it after the latter is not redundant. This occurred at the first light of sunrise. Zayd ibn Ali read it as bukrata (bakarata), indeclinable (ghayr masrufa), due to its status as a proper noun and its feminine gender, implying that what is intended by it is a specific portion of the beginning of the day.

"A settled punishment."

It settles upon them and persists until it delivers them to the Fire, or it is a punishment that cannot be averted from them, or one that has reached its utmost limit.