What is the ruling on one who swore a solemn oath to fast a thousand days or enter the fire of Hell if he returned to a sin, and then returned to it?

To resolve the issue, one must immediately abandon the sin and offer sincere repentance. If the oath was sworn by Allah or one of His attributes, then an expiation for a broken oath (kaffarat yamin (definition)) is required. If the oath was to perform a certain act, such as "I will fast a thousand days," then this is a "vow of obstinacy" (nadhr (definition) al-lajaj), and its maker has the choice between fulfilling it or offering expiation for it. A Muslim supplicating damnation upon himself is not permissible, and he must seek forgiveness for it. One who repents from a sin is like one who has no sin, and Allah's mercy is vast.