Musnad Ahmad 830:60

Sulayman ibn Dawud al-Tayalisi Abu Dawud narrated to us, Imran narrated to us, from Qatadah, from al-Hasan, from Abu Bakrah, that the Prophet (peace and blessings of Allah be upon him) said: "Fast for the crescent upon seeing it, and break the fast upon seeing it. If it is obscured from you, then complete the count to thirty, and the month is like this, and like this, and like this." And he gestured with his fingers.