Musannaf Ibn Abi Shayba 7:352

Hushaym narrated to us, from Yunus, from al-Hasan, regarding a man who obligated upon himself the fasting of a month. He said: "Whoever specified a known month, let him fast it and observe it consecutively. And if he did not specify a known month and did not intend it, let him begin the days and fast thirty days. If he fasted upon the sighting of the crescent and broke his fast upon its sighting, and it was twenty-nine [days], that suffices him. And if he separated [the days], it is [only] if he began the days [anew]."