Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "Do not precede this month (Ramadan) by one or two days, unless it coincides with the fast of one of you who used to keep it beforehand. Fast upon seeing it and break the fast upon seeing it. If it is clouded over you, then break the fast after completing thirty days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7944