Musannaf Ibn Abi Shayba 7:47

Katheer bin Hisham narrated to us, from Sulaiman bin Abi Dawud, who said: Ata bin Abi Rabah was asked about a man who obligated upon himself the fasting of two consecutive months, then the day of Adha or Fitr overtakes him. He said: He should break his fast, then continue [building] upon his fasting.