‘When you see the new crescent, fast, and when you see it, stop fasting. If it is cloudy then calculate it (as thirty days).” Ibn ‘Umar used to fast one day before the new crescent was seen.
قَالَ رَسُولُ اللَّهِ ﷺ إِذَا رَأَيْتُمُ الْهِلاَلَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ وَكَانَ ابْنُ عُمَرَ يَصُومُ قَبْلَ الْهِلاَلِ بِيَوْمٍ