Ibn 'Umar (RAA) narrated, 'The people were trying to sight the new moon (of Ramadan.) and when I reported to the Messenger of Allah (ﷺ) that I had seen it, he fasted and commanded people to fast.' Related by Abu Dawud, and Al-Hakim and Ibn Hibban graded it as Sahih. (ID)
English reference (ID): Book 5, Hadith 675 (ID)
Arabic reference (ID): Book 5, Hadith 654 (ID)