Musnad Ahmad 30:660

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn al-Musayyib and Abu Salamah, or from one of them, 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."