The Book of Fasting
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا إِسْمَاعِيلُ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، - رضى الله عنهما - قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّمَا الشَّهْرُ تِسْعٌ وَعِشْرُونَ فَلاَ تَصُومُوا حَتَّى تَرَوْهُ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا لَهُ " .Ibn'Umar (Allah be pleased with-both of them) reported Allah's Messenger (ﷺ) as saying:The month of Ramadan may consist of twenty-nine days. So do not fast till you have sighted it (the new moon) and do not break fast, till you have sighted it (the new moon of Shawwal), and if the sky is cloudy for you, then calculate
From Sahih Muslim, whose reports the scholars accept as authentic.