Shahih Muslim · #2501
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 " .
English
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