Musnad Ahmad 25:1656

Rawh narrated to us, Shu'bah narrated to us, 'Amr ibn Murrah narrated to us, from Abu al-Bakhtari, who said: We sighted the crescent moon of the month of Ramadan at Dhat 'Irq, so we sent [a messenger] to Ibn 'Abbas to ask him. He said: The Prophet of Allah, may Allah bless him and grant him peace, said: "Indeed, Allah, the Almighty and Majestic, has extended it for its sighting; so if it is obscured from you, then complete the count."