Musnad Ahmad 30:463

Abu Kamil narrated to us, Ibrahim narrated to us, Ibn Shihab narrated to us, from Sa'id ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "When you see the crescent, then fast, and when you see it, then break your fast. But if it is obscured from you, then fast thirty days."