Kitáb-i-Aqdas

Question: Should a person wish to fast at a time other than in the month of 'Alá', is this permissible or not; and if he hath vowed or pledged himself to such a fast, is this valid and acceptable?

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.