Sayyiduna Abu Hurayrah (may Allah be pleased with him) says: "The month is twenty-nine days, so do not fast until you see it, and do not break the fast until you see it. If it is clouded over for you, then complete the count of thirty. Your Eid al-Fitr is the day on which you break the fast, your Adha is the day on which you sacrifice, all of Arafat is a place of standing, all of Mina is a place of sacrifice, and every street of Makkah is a place of sacrifice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8206