Sahih Muslim 1080 p
Sa'd b. 'Ubaida reported that Ibn'Umar (Allah be pleased with both of them) heard a person saying:
This night is the midnight (of the month). Upon this he said to him: How do you know that it is the midnight (of the month), for I heard the Messenger of Allah (ﷺ) as saying: The month is thus and thus (and he pointed with his ten fingers twice) and thus (i. e. at the third time he pointed with all his fingers but withdrew or folded his thumb)?
حَدَّثَنَا أَبُو كَامِلٍ الْجَحْدَرِيُّ، حَدَّثَنَا عَبْدُ الْوَاحِدِ بْنُ زِيَادٍ، حَدَّثَنَا الْحَسَنُ بْنُ عُبَيْدِ، اللَّهِ عَنْ سَعْدِ بْنِ عُبَيْدَةَ، قَالَ سَمِعَ ابْنُ عُمَرَ، - رضى الله عنهما - رَجُلاً يَقُولُ اللَّيْلَةَ لَيْلَةُ النِّصْفِ فَقَالَ لَهُ مَا يُدْرِيكَ أَنَّ اللَّيْلَةَ النِّصْفُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ الشَّهْرُ هَكَذَا وَهَكَذَا ‏"‏ ‏.‏ وَأَشَارَ بِأَصَابِعِهِ الْعَشْرِ مَرَّتَيْنِ ‏"‏ وَهَكَذَا ‏"‏ ‏.‏ فِي الثَّالِثَةِ وَأَشَارَ بِأَصَابِعِهِ كُلِّهَا وَحَبَسَ أَوْ خَنَسَ إِبْهَامَهُ ‏"‏ ‏.‏