Musnad Ahmad 30:2428

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Muhammad ibn Ziyad narrated to us, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Fast upon seeing it, and break your fast upon seeing it. But if it is obscured from you, then complete the count to thirty."