What is the ruling on vowing to perform an act of obedience that has no original religious obligation (e.g., visiting the sick)?

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Vows concerning acts of obedience that are not originally obligatory in Sharia, such as visiting the sick or I'tikaf, must still be fulfilled. This is because the vow is a derivative of prescribed acts, and one is bound by what one obligates upon oneself for the sake of righteousness. Evidence includes the hadith, "Whoever vows to obey Allah, let him obey Him," and the severe warning in the Quran against breaking a covenant made with Allah (Quran 9:75-77). Umar's vow to perform I'tikaf in the Sacred Mosque was confirmed by the Prophet, peace be upon him, who commanded him to fulfill it.