Musnad Ahmad 30:2725

Hajjaj narrated to us, he said: Shu'bah narrated to us, from Muhammad ibn Ziyad, he said: I heard Abu Hurairah, he said: The Messenger of Allah (may Allah bless him and grant him peace) said — or Abu al-Qasim (may Allah bless him and grant him peace) said: "Fast upon seeing it, and break your fast upon seeing it. But if it is obscured from you, then complete thirty [days]."