Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) commanded us that "we fast upon sighting the crescent and break the fast upon sighting it, and if it is clouded over for us, that we complete thirty days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8184