Musannaf Ibn Abi Shaybah 12902 — A man who obligates himself to fast

Al-Hasan said concerning a man who made a month's fasting binding upon himself: If he named a specific month, let him fast it and fast it consecutively. If he did not name a specific month, or did not intend one, let him count the days and fast thirty days. If he began fasting upon (sighting) the new moon and ended his fast upon sighting it (again), and it came to twenty-nine days, that suffices him. And (it is valid) even if he fasts them separately, when he counts the days.