Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not precede this month; rather, fast upon its appearing and break the fast upon its appearing. If clouds or the like cover it over you, then complete the count of thirty."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7945