It hath been ordained that every believer in God, the Lord of Judgement, shall, each day, having washed his hands and then his face, seat himself and, turning unto God, repeat “Alláh-u-Abhá” ninety-five times. Such was the decree of the Maker of the Heavens when, with majesty and power, He established Himself upon the thrones of His Names. Perform ye, likewise, ablutions for the Obligatory Prayer; this is the command of God, the Incomparable, the Unrestrained.

Perform ye . . . ablutions for the Obligatory Prayer
Ablutions are specifically associated with certain prayers. They must precede the offering of the three Obligatory Prayers, the daily recitation of “Alláh-u-Abhá” ninety-five times, and the recital of the verse prescribed as an alternative to obligatory prayer and fasting for women in their courses (see note 20).
The prescribed ablutions consist of washing the hands and the face in preparation for prayer. In the case of the medium Obligatory Prayer, this is accompanied by the recitation of certain verses (see Some Texts Revealed by Bahá’u’lláh Supplementary to the Kitáb-i-Aqdas).
That ablutions have a significance beyond washing may be seen from the fact that even should one have bathed oneself immediately before reciting the Obligatory Prayer, it would still be necessary to perform ablutions (Q&A 18).
When no water is available for ablutions, a prescribed verse is to be repeated five times (see note 16), and this provision is extended to those for whom the use of water would be physically harmful (Q&A 51).
The detailed provisions of the law concerning ablutions are set out in the Synopsis and Codification, section IV.A.10.a.-g., as well as in Questions and Answers numbers 51, 62, 66, 77 and 86.

Question: If, for another purpose, one hath performed ablutions, and the time of obligatory prayer arriveth, are these ablutions sufficient or must they be renewed?
Answer: These same ablutions are sufficient, and there is no need for them to be renewed.

Question: Are ablutions for the morning prayer still valid for the noonday prayer? And similarly, are ablutions carried out at noon still valid in the evening?
Answer: Ablutions are connected with the Obligatory Prayer for which they are performed, and must be renewed for each prayer.

Question: At noon, which is the time for two of the Obligatory Prayers -- the short midday prayer, and the prayer to be offered in the morning, noon, and evening -- is it necessary in this case to perform two ablutions or would one suffice?
Answer: The renewal of ablutions is unnecessary.