Musnad Abu Ya'la al-Mawsili 110:203

Abu Bakr narrated to us, Husayn bin Ali narrated to us, from Za'idah, from Simak bin Harb, from Ikrimah, from Ibn Abbas, who said: A Bedouin came to the Prophet, may Allah bless him and grant him peace, and said: "I saw the crescent moon tonight." He said: "Do you bear witness that there is no god but Allah and that Muhammad is His servant and His messenger?" He said: "Yes." He said: "Stand up, O Bilal, and announce to the people that they should fast tomorrow."