Musnad Ahmad 30:2757

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Muhammad ibn Ziyad, he said: I heard Abu Hurairah narrating that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Do not fast until you see the crescent, and do not break your fast until you see the crescent." Or he said: "Fast for its sighting, and break your fast for its sighting; but if it is obscured from you, then count thirty [days]." Shu'bah said: And the extent of my knowledge is that he said: "Do not fast until you see the crescent, and do not break your fast until you see the crescent."