Musnad Ahmad 25:1615

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, 'Amr ibn Dinar informed me that he heard Muhammad ibn Jubayr say: Ibn 'Abbas used to disapprove of advancing the fasting of Ramadan if the crescent of the month of Ramadan was not seen. And he would say: The Prophet, may Allah bless him and grant him peace, said: "If you do not see the crescent, then complete thirty nights."