'Abdullah b. 'Umar (Allah be pleased with both of them) reported Messenger of Allah ﷺ as saying: The month (of Ramadan) may consist of twenty nine days;
so when you see the new moon observe fast and when you see (the new moon again at the commencement of the month of Shawwal) then break It, and if the sky is cloudy for you, then calculate it (and complete thirty days).

وَحَدَّثَنِي حُمَيْدُ بْنُ مَسْعَدَةَ الْبَاهِلِيُّ، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، حَدَّثَنَا سَلَمَةُ، - وَهُوَ ابْنُ عَلْقَمَةَ - عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، - رضى الله عنهما - قَالَ قَالَ رَسُولُ اللَّهِ ﷺ " الشَّهْرُ تِسْعٌ وَعِشْرُونَ فَإِذَا رَأَيْتُمُ الْهِلاَلَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ " .