Mishkat al-Masabih 7 Fasting كتاب الصوم 13

عَنْ عُمَرَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَا تَصُومُوا حَتَّى تَرَوُا الْهِلَالَ وَلَا تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ» . وَفِي رِوَايَةٍ قَالَ: «الشَّهْرُ تِسْعٌ وَعِشْرُونَ لَيْلَةً فَلَا تَصُومُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلَاثِينَ»

Ibn ‘Umar reported God’s messenger as saying, “Do not fast till you see the new moon, and do not break your fast till you see it; but if the weather is cloudy calculate when it should appear.” In a version he said, “The month consists of twenty-nine days, but do not fast till you see it, and if the weather is cloudy wait till thirty days of the previous month have passed.” (Bukhari and Muslim.)