What is the required formulation for a valid vow (Nathr)?

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The formula for a vow requires the person to say: 'For Allah, it is incumbent upon me to do such and such' (Lillahi 'alayya an af'ala kadha). If a person states, 'It is incumbent upon me, a vow, such and such' ('Alayya nathr katha), the vow is also binding because the speaker explicitly used the term 'vow' (nathr). Furthermore, conditional statements such as, 'If Allah heals me, then upon me is fasting for a month' constitute a vow.