Musnad Ahmad 33:411

Rawh narrated to us, Zakariya narrated to us, Abu al-Zubayr narrated to us, that he heard Jabir ibn Abdullah saying: The Messenger of Allah, may Allah bless him and grant him peace, 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 count thirty days."