صَحِيحُ مُسْلِمٍ
Sahih Muslim 1081 c
The Book of Fasting
The obligation to fast Ramadan when the crescent is sighted, and to break the fast when the crescent is sighted, and that if it is cloudy at the beginning or end of the month, then the month should be completed as thirty days.
وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ مُحَمَّدِ بْنِ زِيَادٍ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ، - رضى الله عنه - يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ غُمِّيَ عَلَيْكُمُ الشَّهْرُ فَعُدُّوا ثَلاَثِينَ .
Abu Huraira reported Allah's Messenger (ﷺ) as saying: Observe fast on sighting it (the new moon) and break it on sighting it. But if (due to clouds) the actual position of the month is concealed from you, you should then count thirty (days).
Sahih Muslim, 1081 cSahihصحيح