What is the expiation for someone who vowed not to do something specific for a month, then did it before the month was over?

A Muslim must fulfill his vow as long as it is not an act of disobedience, and the vowed act must be an act of worship, for Allah Almighty says: ﴿And let them fulfill their vows﴾. If the one who makes the vow specifies something to do, such as fasting or giving charity, when its condition is met, then he must fulfill it. But if he does not name any specific act of obedience, then he must offer expiation for an oath, for the Prophet, peace and blessings be upon him, said: "The expiation for a vow is the expiation for an oath." This involves feeding ten poor persons, or clothing them, or emancipating a slave. If he cannot find any of these, then he must fast for three days.