Al-Aswad bin Yazid, Alqamah and Masruq said that Sayyiduna Abdullah (may Allah be pleased with him) said: "Fasting is from the sighting of the crescent moon to (the next) sighting of it; if it is hidden from you, then (complete) thirty days."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9529