Musannaf Ibn Abi Shayba 4:161

Ibn Fudayl narrated to us, from Husayn, from ‘Amr ibn Murrah, from Abu al-Bakhtari, who said: We went out for ‘Umrah. When we descended at Batn Nakhla, we sighted the crescent moon. Some of the people said, "It is three nights old," and some of the people said, "It is two nights old." We met Ibn ‘Abbas and said, "We sighted the crescent moon, and some of the people said it is three nights old, while some of the people said it is two nights old." He said, "On which night did you see it?" We said, "On such and such a night." He said, "Indeed, the Messenger of Allah (may Allah bless him and grant him peace) said: 'Verily, Allah extended it for the sake of sighting, so it belongs to the night you saw it.'"