A man said, "Tonight is the middle of the month."He replied, "How do you know it is the middle? It is rather the fifteenth. I heard the Messenger of Allah ﷺ say, 'The month is like this, and like this, and like this,'" and on the third time Abu Khalid closed in his thumb to make it fifty.
حدثنا سليمان بن حيان عن الحسن يعني ابن عبيد الله عن سعد بن عبيدة
رجلا يقول الليلة النصف فقال وما يدريك أنها النصف بل خمس عشرة سمعت رسول الله ﷺ يقول الشهر هكذا وهكذا وهكذا وضم أبو خالد في الثالثة خمسين
Its isnad is sahih. Muslim related it in 1:299 through Abdul-Wahid bin Ziyad, from Hasan bin Ubaydullah.As for his statement, "and Abu Khalid added fifty in the third," Abu Khalid is Sulayman bin Hayan, the teacher of Ahmad. The meaning is that he gestured with his four fingers, excluding the thumb. This is clarified by Muslim's version, "and he gestured with his ten fingers twice, and likewise in the third time, and he gestured with all his fingers, and folded back or bent his thumb."The meaning of Ibn Umar's answer, as Nawawi said in 7:193, is that you cannot know for certain whether that night is indeed the fifteenth or not, because the month may sometimes be twenty-nine days. You meant the night of the day whose completion makes up the fifteenth, but this only holds true on the assumption that the month is complete, and you do not know whether it is complete or not.See 6041.