Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "Do not fast before Ramadan. Fast upon sighting it (the crescent) and end the fast upon sighting it, and if clouds come in between, complete thirty days."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11656