The Prophet ﷺ said: Do not fast one day or two days just before Ramadan except in the case of a man who has been in the habit or observing a fast (on that day); and do not fast until you sight it (the moon). Then fast until you sight it. If a cloud appears on that day (i.e. 29th of Ramadan) then complete the number thirty (days) and then end the fasting: a month consists of twenty-nine days.

قَالَ رَسُولُ اللَّهِ ﷺ لاَ تُقَدِّمُوا الشَّهْرَ بِصِيَامِ يَوْمٍ وَلاَ يَوْمَيْنِ إِلاَّ أَنْ يَكُونَ شَىْءٌ يَصُومُهُ أَحَدُكُمْ وَلاَ تَصُومُوا حَتَّى تَرَوْهُ ثُمَّ صُومُوا حَتَّى تَرَوْهُ فَإِنْ حَالَ دُونَهُ غَمَامَةٌ فَأَتِمُّوا الْعِدَّةَ ثَلاَثِينَ ثُمَّ أَفْطِرُوا وَ الشَّهْرُ تِسْعٌ وَعِشْرُونَ

[Machine] The Messenger of Allah ﷺ said, "Do not begin the month by fasting a day or two unless it is something that one of you regularly fasts, and do not fast until you see it (the crescent moon), then fast until you see it (the crescent moon). If it is cloudy before you, then complete the count to thirty and then break your fast (to end the month of Sha'ban)." Abu Dawood narrated this hadith, as did Hatim ibn Abi Sajirah, Shu'bah, and Al-Hasan ibn Salih, all of whom narrated from Simak. However, they did not mention the phrase "then break your fast." The Sheikh (Al-Albani) narrated this hadith from Abu Awanah, summarizing it, and added the phrase "then break your fast."

قَالَ رَسُولُ اللهِ ﷺ لَا تَقَدَّمُوا الشَّهْرَ بِصِيَامِ يَوْمٍ وَلَا يَوْمَيْنِ إِلَّا أَنْ يَكُونَ شَيْئًا يَصُومُهُ أَحَدُكُمْ وَلَا تَصُومُوا حَتَّى تَرَوْهُ ثُمَّ صُومُوا حَتَّى تَرَوْهُ فَإِنْ حَالَ دُونَهُ غَمَامَةٌ فَأَتِمُّوا الْعِدَّةَ ثَلَاثِينَ ثُمَّ أَفْطِرُوا الشَّهْرُ تِسْعٌ وَعِشْرُونَ قَالَ أَبُو دَاوُدَ وَرَوَاهُ حَاتِمُ بْنُ أَبِي صَغِيرَةَ وَشُعْبَةُ وَالْحَسَنُ بْنُ صَالِحٍ عَنْ سِمَاكٍ بِمَعْنَاهُ لَمْ يَقُولُوا ثُمَّ أَفْطِرُوا قَالَ الشَّيْخُ وَرَوَاهُ أَبُو عَوَانَةَ عَنْ سِمَاكٍ مُخْتَصَرًا فَجَعَلَ إِكْمَالَ الْعِدَّةِ لِشَعْبَانَ