Musannaf Ibn Abi Shayba 7:46

Shababa ibn Sawwar narrated to us, from Shu'bah, who said: I asked al-Hakam and Hammad about a woman who vowed to fast every Friday, and that day coincided with a day of Fitr or Adha. They both said: She shall make up a day in its place and offer expiation for it.