It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) mentioned the crescent and said: "Fast upon its appearing and break the fast upon its appearing; if it is clouded over you, then complete thirty days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7935