God hath decreed, in token of His mercy unto His creatures, that semen is not unclean. Yield thanks unto Him with joy and radiance, and follow not such as are remote from the Dawning-place of His nearness. Arise ye, under all conditions, to render service to the Cause, for God will assuredly assist you through the power of His sovereignty which overshadoweth the worlds. Cleave ye unto the cord of refinement with such tenacity as to allow no trace of dirt to be seen upon your garments. Such is the injunction of One Who is sanctified above all refinement. Whoso falleth short of this standard with good reason shall incur no blame. God, verily, is the Forgiving, the Merciful. Wash ye every soiled thing with water that hath undergone no alteration in any one of the three respects; take heed not to use water that hath been altered through exposure to the air or to some other agent. Be ye the very essence of cleanliness amongst mankind. This, truly, is what your Lord, the Incomparable, the All-Wise, desireth for you.

Wash ye every soiled thing with water that hath undergone no alteration in any one of the three respects
The “three respects” referred to in this verse are changes in the colour, taste or smell of the water. Bahá’u’lláh provides additional guidance concerning pure water and the point at which it is considered unsuitable for use (Q&A 91).

Question: Concerning pure water, and the point at which it is considered used.
Answer: Small quantities of water, such as one cupful, or even two or three, must be considered used after a single washing of the face and hands. But a kurr [this refers to a volume of approximately one half of a cubic metre] or more of water remaineth unchanged after one or two washings of the face, and there is no objection to its use unless it is altered in one of the three ways [colour, taste and smell], for example its colour is changed, in which case it should be looked upon as used.