Musnad Abu Ya'la al-Mawsili 159:5

Abu Bakr narrated to us, Muhammad ibn Bishr narrated to us, from Ubayd Allah, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, who said: The crescent moon was mentioned to the Messenger of Allah, may Allah bless him and grant him peace, and he said: "When you see it, then fast, and when you see it, then break your fast. But if it is obscured from you, then count thirty."