Musnad Ahmad 30:746

Muhammad ibn Bishr narrated to us, Ubaydullah ibn Umar narrated to us, from Abu al-Zinad, from al-A’raj, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, mentioned the crescent moon and 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 [days]."