The Kitáb-i-Aqdas - The Most Holy Book
Index term: Adultery
k19.
Ye have been forbidden to commit murder or adultery, or to engage in backbiting or calumny; shun ye, then, what hath been prohibited in the holy Books and Tablets.
k49.
God hath imposed a fine on every adulterer and adulteress, to be paid to the House of Justice: nine mithqáls of gold, to be doubled if they should repeat the offence. Such is the penalty which He Who is the Lord of Names hath assigned them in this world; and in the world to come He hath ordained for them a humiliating torment. Should anyone be afflicted by a sin, it behoveth him to repent thereof and return unto his Lord. He, verily, granteth forgiveness unto whomsoever He willeth, and none may question that which it pleaseth Him to ordain. He is, in truth, the Ever-Forgiving, the Almighty, the All-Praised.
n36.
or adultery
The Arabic word "ziná", here translated as "adultery", signifies both fornication and adultery. It applies not only to sexual relations between a married person and someone who is not his or her spouse, but also to extramarital sexual intercourse in general. One form of "ziná" is rape. The only penalty prescribed by Bahá'u'lláh is for those who commit fornication (see note 77); penalties for other kinds of sexual offence are left to the Universal House of Justice to determine.
n77.
God hath imposed a fine on every adulterer and adulteress, to be paid to the House of Justice
Although the term translated here as adultery refers, in its broadest sense, to unlawful sexual intercourse between either married or unmarried individuals (see note 36 for a definition of the term), 'Abdu'l-Bahá has specified that the punishment here prescribed is for sexual intercourse between persons who are unmarried. He indicates that it remains for the Universal House of Justice to determine the penalty for adultery committed by a married individual. (See also Q&A 49.)
In one of His Tablets, 'Abdu'l-Bahá refers to some of the spiritual and social implications of the violation of the laws of morality and, concerning the penalty here described, He indicates that the aim of this law is to make clear to all that such an action is shameful in the eyes of God and that, in the event that the offence can be established and the fine imposed, the principal purpose is the exposure of the offenders -- that they are shamed and disgraced in the eyes of society. He affirms that such exposure is in itself the greatest punishment.
The House of Justice referred to in this verse is presumably the Local House of Justice, currently known as the Local Spiritual Assembly.
q23.
 
Question: Concerning the punishment of the adulterer and adulteress.
Answer: Nine mithqáls are payable for the first offence, eighteen for the second, thirty-six for the third, and so on, each succeeding fine being double the preceding. The weight of one mithqál is equivalent to nineteen nakhuds in accordance with the specification of the Bayán.
q49.
 
Question: Concerning the penalties for adultery, sodomy, and theft, and the degrees thereof.
Answer: The determination of the degrees of these penalties rests with the House of Justice.