Musnad Abu Ya'la al-Mawsili 153:38

Abu Khaythama narrated to us, Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Shihab, from Salim, from his father, who said: The Messenger of Allah (peace be upon him) said: "When you see the crescent, then fast, and when you see it, then break your fast. If it is obscured from you, then estimate it." And Abdullah used to fast one day before the crescent.