ما هي الفتوى فيمن حلف بصيامه على ألا يفعل شيئاً ثم أراد أن يفعله؟

It is not permissible to swear an oath except by Allah or by His names and attributes, for the Prophet, peace and blessings be upon him, said: "Whoever swears an oath, let him swear by Allah or remain silent." Swearing by other than Allah is minor shirk, for the Prophet, peace and blessings be upon him, said: "Whoever swears by other than Allah has committed shirk."

Swearing by fasting, prayer, or any act of obedience is swearing by other than Allah, and it is forbidden. The one who does it must repent, feel remorse, and resolve not to repeat it.

An oath sworn by other than Allah is not binding; there is no obligation to fulfill it, nor is there any expiation for it, because it is invalid and nothing results from it except seeking forgiveness and repentance.

Ibn Hazm said: "Whoever swears by other than Allah is not truly swearing, and it is not an oath. It is invalid, and there is nothing in it except seeking Allah's forgiveness and repentance only."

There is no harm in doing what one swore not to do if it is permissible, and there is no expiation for it, but one must repent from this forbidden oath.