رَوَى مُحَمَّدُ بْنُ مُسْلِمٍ عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ قَالَ: «إِذَا رَأَيْتُمُ اَلْهِلاَلَ فَصُومُوا وَ إِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا وَ لَيْسَ بِالرَّأْيِ وَ اَلتَّظَنِّي وَ لَيْسَ اَلرُّؤْيَةُ أَنْ يَقُومَ عَشَرَةُ نَفَرٍ يَنْظُرُونَ فَيَقُولَ وَاحِدٌ مِنْهُمْ هُوَ ذَا هُوَ ذَا وَ يَنْظُرَ تِسْعَةٌ فَلاَ يَرَوْنَهُ وَ لَكِنْ إِذَا رَآهُ وَاحِدٌ رَآهُ أَلْفٌ».
Muhammad ibn Muslim narrated from Abu Ja'far (a.s), who said: "When you see the crescent, fast, and when you see it again, break your fast. This is not based on conjecture or speculation. Seeing the crescent does not mean that ten people look for it, and one of them says, 'There it is, there it is,' while the other nine do not see it. Rather, when one person sees it, it is as though a thousand have seen it."