Musnad Ahmad 30:398

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu Salamah ibn 'Abd al-Rahman, from Abu Hurayrah, that 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."