Abu al-Bakhtari (may Allah have mercy on him) said: We set out for Hajj, and when we stopped at Batn Nakhl we saw the new crescent moon. Some people said: It is three nights old, and some said: It is two nights old. We came to Ibn Abbas (may Allah be pleased with them both) and said: We saw the new crescent moon; some people said: It is three nights old, and some said: It is two nights old. He said: "The Messenger of Allah (peace and blessings be upon him) extended (the month) until it is sighted, so it belongs to the night on which you saw it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12633