Yahya bin Yahya (may Allah have mercy on him) says: I read before Malik (may Allah have mercy on him), with the chain of Nafi' (may Allah have mercy on him), from Ibn Umar (may Allah be pleased with them both), from the Prophet (peace and blessings be upon him), the hadith that he (peace and blessings be upon him) mentioned Ramadan and said: "Do not fast until you see the new moon, and do not break the fast until you see it. If it is clouded over for you, then estimate (the days) for it."
In the narration of al-Qa'nabi (may Allah have mercy on him): the Messenger of Allah (peace and blessings be upon him) mentioned Ramadan and said: "If it is clouded over for you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7922