The Kitáb-i-Aqdas - The Most Holy Book
Index term: of feet, under Washing
k152.
Wash your feet once every day in summer, and once every three days during winter.
n167.
Wash your feet
The believers are exhorted in the Kitáb-i-Aqdas to bathe regularly, to wear clean clothes and generally to be the essence of cleanliness and refinement. The Synopsis and Codification, section IV.D.3.y.i.-vii., summarizes the relevant provisions. In relation to the washing of the feet, Bahá'u'lláh states that it is preferable to use warm water; however, washing in cold water is also permissible (Q&A 97).
q97.
 
Question: Concerning the washing of the feet in winter and summer.
Answer: It is the same in both cases; warm water is preferable, but there can be no objection to cold.