Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Fast upon its appearing and break the fast upon its appearing. If in that month it is clouded over you, then count thirty days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7933