Mishkat al-Masabih 7 Fasting كتاب الصوم 14

وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ غم عَلَيْكُم فَأَكْمِلُوا عِدَّةَ شَعْبَانَ ثَلَاثِينَ»

Abu Huraira reported God’s messenger as saying, “Fast when you see it and break your fast when you see it, and if the weather is cloudy treat Sha‘ban as having thirty days.” (Bukhari and Muslim.)