Qays bin Talq (may Allah have mercy on him) narrated from his father that he said: I heard a man asking the Prophet (peace and blessings be upon him) about the day of doubt, some of them saying it is Sha'ban and some saying it is Ramadan. The Messenger of Allah (peace and blessings be upon him) said: "Do not fast until you see the crescent, and do not break the fast until you see the crescent; and if it is clouded over you, then complete the count of thirty."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7951