Abu Huraira (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: When you see the moon, then fast, and when you see the moon, then break the fast. If the sky is cloudy, then complete thirty days of fasting.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصيام / 395