Sahih al-Bukhari 1907
Sahih al-Bukhari · Book 30: Fasting · "Start observing Saum on seeing the crescent of Ramadan, and stop on seeing the crescent (of Shawwal)."
Narrated `Abdullah bin `Umar:
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ ، حَدَّثَنَا مَالِكٌ ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " الشَّهْرُ تِسْعٌ وَعِشْرُونَ لَيْلَةً، فَلاَ تَصُومُوا حَتَّى تَرَوْهُ، فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاَثِينَ " .
Allah's Messenger (ﷺ) said, "The month (can be) 29 nights (i.e. days), and do not fast till you see the moon, and if the sky is overcast, then complete Sha'ban as thirty days."