The Messenger of Allah ﷺ used to count the days in Sha'ban in a manner he did not count any other month; then he fasted when he sighted the new moon of Ramadan; but if the weather was cloudy he counted thirty days and then fasted.  

سَمِعْتُ عَائِشَةَ ؓ تَقُولُ كَانَ رَسُولُ اللَّهِ ﷺ يَتَحَفَّظُ مِنْ شَعْبَانَ مَا لاَ يَتَحَفَّظُ مِنْ غَيْرِهِ ثُمَّ يَصُومُ لِرُؤْيَةِ رَمَضَانَ فَإِنْ غُمَّ عَلَيْهِ عَدَّ ثَلاَثِينَ يَوْمًا ثُمَّ صَامَ  

The Prophet ﷺ said: Do not fast (for Ramadan) before the coming of the month until you sight the moon or complete the number (of thirty days); then fast until you sight the moon or complete the number (of thirty days).  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ تُقَدِّمُوا الشَّهْرَ حَتَّى تَرَوُا الْهِلاَلَ أَوْ تُكْمِلُوا الْعِدَّةَ ثُمَّ صُومُوا حَتَّى تَرَوُا الْهِلاَلَ أَوْ تُكْمِلُوا الْعِدَّةَ