وَعَنِ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا
سَمِعْتُ رَسُولَ اَللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «إِذَا رَأَيْتُمُوهُ فَصُومُوا، وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا، فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ»
‘I heard the Messenger of Allah (sallallahu alayhi wa sallam) say, ‘Start fasting after you have seen it (i.e. the new crescent) and end the fast (at the end of the month) when you see it. If it is overcast and cannot be seen, then estimate its sighting (i.e. presume that Shaban lasted for thirty days).’ Agreed upon.
Muslim narrated, ‘If it is too overcast to be seen then presume that it (Shaban) lasted for thirty days.’In another narration by al-Bukhari, ‘wait, until thirty days of it (Shaban) have passed.’’
Bulugh al-Maram 760, Book 5, Fasting, Hadith 3. 1 min read