Ibn Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:

The month is thus and thus and thus (i. e. pointing with his fingers thrice), and he held back his thumb at the third time (in order to show that it can also consist of twenty-nine days).

حَدَّثَنَا هَارُونُ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا رَوْحُ بْنُ عُبَادَةَ، حَدَّثَنَا زَكَرِيَّاءُ بْنُ إِسْحَاقَ، حَدَّثَنَا عَمْرُو بْنُ دِينَارٍ، أَنَّهُ سَمِعَ ابْنَ عُمَرَ، - رضى الله عنهما - يَقُولُ سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ ‏

‏ الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا ‏

‏ ‏.‏ وَقَبَضَ إِبْهَامَهُ فِي الثَّالِثَةِ ‏.‏