Musnad Abu Ya'la al-Mawsili 109:249

Abu Khaythama narrated to us, Rawh narrated to us, Zakariyya ibn Ishaq narrated to us, Abu al-Zubayr narrated to us, that he heard Jabir 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."