Ibn 'Umar (RAA) narrated, ‘I heard the Messenger of Allah (ﷺ) 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 Sha‘ban lasted for thirty days)." Agreed upon. Muslim narrated, "If it is too overcast to be seen then presume that it (Sha'ban) lasted for thirty days."In another narration by al-Bukhari, "wait, until thirty days of it (Sha’ban) have passed."

وَعَنِ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا [ قَالَ ]: سَمِعْتُ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-يَقُولُ: { إِذَا رَأَيْتُمُوهُ فَصُومُوا, وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا, فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ } مُتَّفَقٌ عَلَيْهِ [1]‏ .‏ وَلِمُسْلِمٍ: { فَإِنْ أُغْمِيَ عَلَيْكُمْ فَاقْدُرُوا [ لَهُ ] [2]‏ .‏ ثَلَاثِينَ } [3]‏ .‏ وَلِلْبُخَارِيِّ: { فَأَكْمِلُوا اَلْعِدَّةَ ثَلَاثِينَ } [4]‏ .‏