Musnad Ahmad 25:1162

Muhammad ibn Ja‘far and Hashim narrated to us, they said: Shu‘bah narrated to us, from ‘Amr ibn Murrah, he said: I heard Abu al-Bakhtari say: We sighted the crescent of Ramadan while we were at Dhat ‘Irq. He said: So we sent a man to Ibn ‘Abbas to ask him—Hashim said: So he asked him—and Ibn ‘Abbas said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, Allah has extended its sighting—Hashim said: for its sighting—so if it is obscured from you, then complete the count."