Musnad Abu Ya'la al-Mawsili 110:28

Khalf ibn Hisham narrated to us, Abu al-Ahwas narrated to us, from Simak, from Ikrimah, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Do not fast before Ramadan. Fast upon seeing it (the crescent), and break your fast upon seeing it. If a cloud cover (ghayah) obscures it from you, then complete thirty [days]."