صَحِيحُ مُسْلِمٍ
Sahih Muslim 1080 d
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 سَعِيدٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ عُبَيْدِ اللَّهِ، بِهَذَا الإِسْنَادِ وَقَالَ ذَكَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم رَمَضَانَ فَقَالَ " الشَّهْرُ تِسْعٌ وَعِشْرُونَ الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا " . وَقَالَ " فَاقْدِرُوا لَهُ " . وَلَمْ يَقُلْ " ثَلاَثِينَ " .
'Ubaidullah narrated on the authority of the same chain of transmitters that the Messenger of Allah (ﷺ) made a mention of Ramadan and said: The month may consist of twenty-nine days, and it may be thus, thus and thus, and (he further) said: Calculate it, but he did not say thirty.
Sahih Muslim, 1080 dSahihصحيح