Musnad Ahmad 30:2253

'Affan narrated to us, Hammad ibn Salamah narrated to us, Muhammad ibn Ziyad informed us, he said: I heard Abu Hurairah saying: I heard Abu al-Qasim (may Allah bless him and grant him peace) saying: "Fast for the crescent by seeing it, and break the fast by seeing it. But if it is obscured from you, then complete thirty [days]."