Musnad Ahmad 25:1349

Waki’ narrated to us, Shu’bah narrated to us, from ‘Amr ibn Murrah, from Abu al-Bakhtari, who said: We sighted the crescent moon of Ramadan at Dhat ‘Irq, so we sent a man to Ibn ‘Abbas, and he asked him. He said: "Indeed, the Messenger of Allah (may Allah bless him and grant him peace) extended it to its sighting."