Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Fast upon sighting it and break the fast upon sighting it. If it is clouded over for you, then fast thirty days." [Sahih. See the previous one.]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8183