Musnad Abu Ya'la al-Mawsili 110:61

Zuhayr narrated to us, Ibn 'Uyaynah narrated to us, from 'Amr, from Muhammad ibn Jubayr, from Ibn 'Abbas, who said: I am amazed at those who precede the month, when the Messenger of Allah (peace and blessings of Allah be upon him) has said: "When you see it, then fast, and when you see it, then break your fast. But if it is obscured from you, then complete the count to thirty."