Sahih Muslim Book - 13, Hadith - 2499

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِى شَيْبَةَ حَدَّثَنَا أَبُو أُسَامَةَ حَدَّثَنَا عُبَيْدُ اللَّهِ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ رضى الله عنهما أَنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- ذَكَرَ رَمَضَانَ فَضَرَبَ بِيَدَيْهِ فَقَالَ « الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا - ثُمَّ عَقَدَ إِبْهَامَهُ فِى الثَّالِثَةِ - فَصُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ أُغْمِىَ عَلَيْكُمْ فَاقْدِرُوا لَهُ ثَلاَثِينَ ».

It was narrated from Ibn Umar [May Allah pleased with them] that the Prophet (SAW) mentioned Ramadan and he gestured with his hands and said: “The month is like this and like this and like this,” and he tucked his thumb away the third time. “Fasts when you see it (the crescent), and break the fast when you see it, and if it is cloudy, then count thirty for it.”