Musannaf Ibn Abi Shayba 7:82

Narrated to us Abd al-Rahim, from Ash'ath, from al-Hasan, regarding a woman who made it incumbent upon herself to perform i'tikaf, and then menstruation overtook her: She shall make up the number of days she menstruated. [Regarding] one who swears to fast three days as an expiation for an oath, then menstruates: [she shall make them up].