Sahih Bukhari Volume 003, Book 031, Hadith Number 207.
Narated By Abu Aiyub from Juwairiya bint Al-Harith : The Prophet visited her (Juwairiya) on a Friday and she was fasting. He asked her, "Did you fast yesterday?" She said, "No." He said, "Do you intend to fast tomorrow?" She said, "No." He said, "Then break your fast." Through another series of narrators, Abu Aiyub is reported to have said, "He ordered her and she broke her fast."