كتاب الصيام
13  
The Book of Fasting
(2)
Chapter: 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.
(2)
بَاب وُجُوبِ صَوْمِ رَمَضَانَ لِرُؤْيَةِ الْهِلَالِ وَالْفِطْرِ لِرُؤْيَةِ الْهِلَالِ وَأَنَّهُ إِذَا غُمَّ فِي أَوَّلِهِ أَوْ آخِرِهِ أُكْمِلَتْ عِدَّةُ الشَّهْرِ ثَلَاثِينَ يَوْمًا
Sahih Muslim 1080 o

This hadith has been narrated on the authority of Aswad b. Qais with the same chain of transmitters, but herein no mention has been made of the other month (consisting of) thirty days.

وَحَدَّثَنِيهِ مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا ابْنُ مَهْدِيٍّ، عَنْ سُفْيَانَ، عَنِ الأَسْوَدِ بْنِ قَيْسٍ، بِهَذَا الإِسْنَادِ وَلَمْ يَذْكُرْ لِلشَّهْرِ الثَّانِي ثَلاَثِينَ ‏.‏
Reference : Sahih Muslim 1080o
In-book reference : Book 13, Hadith 18
USC-MSA web (English) reference : Book 6, Hadith 2376
  (deprecated numbering scheme)