Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: The Messenger of Allah (peace and blessings be upon him) said: "Do not fast before Ramadan. Fast upon sighting it and break your fast upon sighting it, and if clouds come in between, then complete thirty days."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11706