عَنْ عَبْدِ اللهِ بْنِ عُمَرَ رَضِيَ اللهُ عَنْهُمَا، قَالَ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: الشَّهْرُ تِسْعٌ وَعِشْرُونَ، فَإِذَا رَأَيْتُمُ الْهِلَالَ فَصُومُوا، وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا، فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ(مسلم:۱۰۸۰)
Translation
It is narrated from Hazrat Abdullah bin Umar (R.A), he said: The Messenger of Allah (ﷺ) said: "The month is also of twenty-nine (days), so when you see the moon, observe the fast, and when you see it, end the fasts, if clouds overshadow you, then complete its counting."