Narrated Ibn 'Umar:

The Prophet (holding out his ten fingers thrice), said, "The month is thus and thus and thus," namely thirty days. Then (holding out his ten fingers twice and then nine fingers), he said, "It may be thus and thus and thus," namely twenty nine days. He meant once thirty days and once twenty nine days.

حَدَّثَنَا آدَمُ، حَدَّثَنَا شُعْبَةُ، حَدَّثَنَا جَبَلَةُ بْنُ سُحَيْمٍ، سَمِعْتُ ابْنَ عُمَرَ، يَقُولُ قَالَ النَّبِيُّ ﷺ " الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا ". يَعْنِي ثَلاَثِينَ، ثُمَّ قَالَ " وَهَكَذَا وَهَكَذَا وَهَكَذَا ". يَعْنِي تِسْعًا وَعِشْرِينَ يَقُولُ، مَرَّةً ثَلاَثِينَ وَمَرَّةً تِسْعًا وَعِشْرِينَ.