Mishkat al-Masabih 7 Fasting كتاب الصوم 23

وَعَنِ ابْنِ عُمَرَ قَالَ: تَرَاءَى النَّاسُ الْهِلَالَ فَأَخْبَرْتُ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِنِّي رَأَيْتُهُ فَصَامَ وَأَمَرَ النَّاسَ بِصِيَامِهِ. رَوَاهُ أَبُو دَاوُد والدارمي

Ibn ‘Umar said that the people tried to see the new moon and he informed God’s messenger that he had seen it, so he fasted and commanded the people to observe the fast.” Abu Dawud and Darimi transmitted it.