What is the ruling on one who made a vow during Eid al-Adha and did not fulfill it?

Scholars have unanimously agreed on the permissibility of fulfilling a promise, and the majority have deemed it recommended. If the promiser emphasizes his promise with an oath, and the promised matter is an act of righteousness and obedience, then the recommendation to fulfill the promise is further confirmed. If he fulfills it, he has done what is best, and there is no expiation upon him. If he does not fulfill it, expiation becomes obligatory upon him.