What is the ruling on one who made a covenant with Allah not to be late for congregational prayer, but then was late for it by mistake, not intentionally, due to negligence?

Scholars differed on the wording "I covenant with Allah." The Hanafis and Hanbalis consider it an oath. The Malikis have two opinions, the more prominent of which is that it is not an oath. The Shafi'is consider it an oath if intended as such.

The most preponderant view is that it can be both a vow and an oath if it entails an act of obedience and devotion (such as Hajj), and an oath only if it entails something that is not an act of devotion (such as not speaking to someone).

Whatever was a vow must be fulfilled. Whatever was an oath, the one who made it has the choice between fulfilling it or offering expiation (if it was not a sin).

The covenant with Allah is great. Expiation may be necessary if one intentionally misses congregational prayer. If the oath was to not miss it intentionally or unintentionally, expiation becomes immediately obligatory due to the impossibility of fulfilling the oath.