What is the expiation for one who breaks his oath not to masturbate?

The might (Izzat) of Allah is one of His attributes by which oaths are bound. Scholars have unanimously agreed that an oath is bound by Allah, by His Most Beautiful Names, and by His attributes. Whoever swears an oath to abandon forbidden acts, such as masturbation, is obligated to fulfill his oath. If he returns to it, he has broken his oath, and he must repent and offer expiation for the oath: 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.