The Kitáb-i-Aqdas - The Most Holy Book
Index term: time for Fast, under Sunrise
k17.
These are the ordinances of God that have been set down in the Books and Tablets by His Most Exalted Pen. Hold ye fast unto His statutes and commandments, and be not of those who, following their idle fancies and vain imaginings, have clung to the standards fixed by their own selves, and cast behind their backs the standards laid down by God. Abstain from food and drink from sunrise to sundown, and beware lest desire deprive you of this grace that is appointed in the Book.
n25.
We have enjoined upon you fasting during a brief period
Fasting and obligatory prayer constitute the two pillars that sustain the revealed Law of God. Bahá'u'lláh in one of His Tablets affirms that He has revealed the laws of obligatory prayer and fasting so that through them the believers may draw nigh unto God.
Shoghi Effendi indicates that the fasting period, which involves complete abstention from food and drink from sunrise till sunset, is

. . . essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, therefore, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires.

Fasting is enjoined on all the believers once they attain the age of 15 and until they reach the age of 70 years.
A summary of the detailed provisions concerning the law of fasting and of the exemptions granted to certain categories of people is contained in the Synopsis and Codification, section IV.B.1.-6. For a discussion of the exemptions from fasting see notes 14, 20, 30 and 31.
The nineteen-day period of fasting coincides with the Bahá'í month of 'Alá', usually 2-20 March, immediately after the termination of the Intercalary Days (see notes 27 and 147), and is followed by the feast of Naw-Rúz (see note 26).
n32.
Abstain from food and drink from sunrise to sundown
This relates to the period of fasting. In one of His Tablets, 'Abdu'l-Bahá, after stating that fasting consists of abstinence from food and drink, further indicates that smoking is a form of "drink". In Arabic the verb "drink" applies equally to smoking.