وَعَنِ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا
قَالَ: «تَرَاءَى اَلنَّاسُ اَلْهِلَالَ، فَأَخْبَرْتُ رَسُولَ اَللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنِّي رَأَيْتُهُ، فَصَامَ، وَأَمَرَ اَلنَّاسَ بِصِيَامِهِ»
‘The people were trying to sight the new moon (of Ramadan.) and when I reported to the Messenger of Allah (sallallahu alayhi wa sallam) 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.
Bulugh al-Maram 762, Book 5, Fasting, Hadith 5. 1 min read