Messenger of Allah ﷺ mentioned Ramadan and said, "Do not fast unless you see the crescent (of Ramadan), and do not give up fasting till you see the crescent (of Shawwal), but if the sky is overcast (if you cannot see it), then act on estimation (i.e. count Sha'ban as 30 days).  

أَنَّ رَسُولَ اللَّهِ ﷺ ذَكَرَ رَمَضَانَ فَقَالَ لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ  

The Prophet (holding out his ten fingers thrice), said, "The month is thus and thus and thus," namely thirty days. Then (holding out his ten fingers twice and then nine fingers), he said, "It may be thus and thus and thus," namely twenty nine days. He meant once thirty days and once twenty nine days.  

قَالَ النَّبِيُّ ﷺ الشَّهْرُ هَكَذَا وَهَكَذَا وَهَكَذَا يَعْنِي ثَلاَثِينَ ثُمَّ قَالَ وَهَكَذَا وَهَكَذَا وَهَكَذَا يَعْنِي تِسْعًا وَعِشْرِينَ يَقُولُ مَرَّةً ثَلاَثِينَ وَمَرَّةً تِسْعًا وَعِشْرِينَ  

The Prophet ﷺ said, "We are an illiterate nation; we neither write, nor know accounts. The month is like this and this, i.e. sometimes of 29 days and sometimes of thirty days."  

عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ إِنَّا أُمَّةٌ أُمِّيَّةٌ لاَ نَكْتُبُ وَلاَ نَحْسُبُ الشَّهْرُ هَكَذَا وَهَكَذَا يَعْنِي مَرَّةً تِسْعَةً وَعِشْرِينَ وَمَرَّةً ثَلاَثِينَ  

that the Prophet ﷺ said or he said Abū al-Qasim said, "˹Start˺ fasting ˹the month˺ upon seeing it (the moon), and break ˹fasting the month˺ upon seeing it. If it is hidden for you (as a result of an overcast at the start of the month), then complete thirty days of Shaʿbān."  

قَالَ النَّبِيُّ ﷺ أَوْ قَالَ قَالَ أَبُو الْقَاسِمِ ﷺ «صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ غُبِّيَ عَلَيْكُمْ فَأَكْمِلُوا عِدَّةَ شَعْبَانَ ثَلاَثِينَ۔»