Abu Hurairah (RAA) narrated that The Messenger of Allah said, "A woman is not to fast (even) for one day while her husband is present except with his permission" Agreed upon and the wording is from Al-Bukhari. Abu Dawud's version states, "unless it is during Ramadan (then she does not need his permission as it is obligatory fasting).'' (IT)
English reference (IT): Book 5, Hadith 705 (IT)
Arabic reference (IT): Book 5, Hadith 685 (IT)