Musnad Abu Ya'la al-Mawsili 153:34

'Abd al-'Aziz ibn Abi Salamah al-'Umari narrated to us, he said: Ibrahim narrated to me, from al-Zuhri, from Salim, from his father, who said: The Messenger of Allah (ﷺ) 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." Salim said: 'Abdullah used to fast a day before the crescent if it was obscured from him.