Sahih Muslim 1080 d
'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.
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 ‏"‏ ‏.‏