ANSWER: The ordinance of fasting is such as hath already
been revealed. Should someone pledge himself, however, to offer up a fast to
God, seeking in this way the fulfilment of a wish, or to realize some other
aim, this is permissible, now as heretofore. Howbeit, it is God’s wish, exalted
be His glory, that vows and pledges be directed to such objectives as will
profit mankind.
- Baha’u’llah (‘Questions and Answers’; ‘The Kitab-i-Aqdas’)