Musnad Ahmad 30:2930

'Abd al-Rahman narrated to us, Hammad narrated to us, from Muhammad ibn Ziyad, who said: I heard Abu Hurayrah saying: I heard Abu al-Qasim (may Allah bless him and grant him peace) saying: "Fast at its sighting, and break your fast at its sighting. But if it is obscured from you, then count thirty."