It was narrated that 'Aishah said:
"The Messenger of Allah ﷺ swore that he would not enter upon his wives for a month, and he stayed for twenty-nine days until, on the eve of the thirtieth, he entered upon me. I said: 'You swore not to enter upon us for a month.' He said: 'The month may be like this,' and he held up his (ten) fingers three times; 'or the month may be like this,' and he held up his fingers three times, keeping one finger down on the third time."

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ أَبِي الرِّجَالِ، عَنْ أَبِيهِ، عَنْ عَمْرَةَ، عَنْ عَائِشَةَ، قَالَتْ أَقْسَمَ رَسُولُ اللَّهِ ﷺ أَنْ لاَ يَدْخُلَ عَلَى نِسَائِهِ شَهْرًا فَمَكَثَ تِسْعَةً وَعِشْرِينَ يَوْمًا حَتَّى إِذَا كَانَ مَسَاءَ ثَلاَثِينَ دَخَلَ عَلَىَّ فَقُلْتُ إِنَّكَ أَقْسَمْتَ أَنْ لاَ تَدْخُلَ عَلَيْنَا شَهْرًا . فَقَالَ " شَهْرٌ هَكَذَا " . يُرْسِلُ أَصَابِعَهُ فِيهَا ثَلاَثَ مَرَّاتٍ " وَشَهْرٌ هَكَذَا " . وَأَرْسَلَ أَصَابِعَهُ كُلَّهَا وَأَمْسَكَ إِصْبَعًا وَاحِدًا فِي الثَّالِثَةِ .