Hadith 5:5
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.
وَعَنِ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا قَالَ: { تَرَاءَى اَلنَّاسُ اَلْهِلَالَ, فَأَخْبَرْتُ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -أَنِّي رَأَيْتُهُ, فَصَامَ, وَأَمَرَ اَلنَّاسَ بِصِيَامِهِ } رَوَاهُ أَبُو دَاوُدَ, وَصَحَّحَهُ اِبْنُ حِبَّانَ, وَالْحَاكِمُ