Sahih al-Bukhari 1909
Narrated Abu Huraira:
The Prophet or Abu-l-Qasim said, "Start fasting on seeing the crescent (of Ramadan), and give up fasting on seeing the crescent (of Shawwal), and if the sky is overcast (and you cannot see it), complete thirty days of Sha'ban."
حَدَّثَنَا آدَمُ، حَدَّثَنَا شُعْبَةُ، حَدَّثَنَا مُحَمَّدُ بْنُ زِيَادٍ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ ـ رضى الله عنه ـ يَقُولُ قَالَ النَّبِيُّ ﷺ أَوْ قَالَ قَالَ أَبُو الْقَاسِمِ ﷺ
" صُومُوا لِرُؤْيَتِهِ، وَأَفْطِرُوا لِرُؤْيَتِهِ، فَإِنْ غُبِّيَ عَلَيْكُمْ فَأَكْمِلُوا عِدَّةَ شَعْبَانَ ثَلاَثِينَ ".
Classification
Sahih (Authentic)
References
Sahih al-Bukhari 1909
Sahih al-Bukhari Vol. 3, Book of Fasting, Hadith 133