(2) بَاب وُجُوبِ صَوْمِ رَمَضَانَ لِرُؤْيَةِ الْهِلَالِ وَالْفِطْرِ لِرُؤْيَةِ الْهِلَالِ وَأَنَّهُ إِذَا غُمَّ فِي أَوَّلِهِ أَوْ آخِرِهِ أُكْمِلَتْ عِدَّةُ الشَّهْرِ ثَلَاثِينَ يَوْمًا
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)?
Reference: Sahih Muslim 1080 p
In-book reference: Book 13, Hadith 19
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2377