[Machine] Ammar ibn Yasir said, "Whoever fasts on the day that is doubted has disobeyed Abu al-Qasim (referring to Prophet Muhammad)."  

عَمَّارُ بْنُ يَاسِرٍ «مَنْ صَامَ الْيَوْمَ الَّذِي يُشَكُّ فِيهِ فَقَدْ عَصَى أَبَا الْقَاسِمِ ﷺ»  

"I entered upon 'Ikrimah on the day concerning which there was doubt as to whether it was Ramadan or Shaban, and he was eating bread, vegetables and milk. He said: 'Come and eat.' I said: 'I am fasting.' He adjured me by Allah to break my fast. I said Subhan-Allah twice. When I saw that he was insisting, I went forward and said: 'Give me what you have.' He said: 'I heard Ibn 'Abbas say: The Messenger of Allah said: 'Fast when you see it (the crescent) and stop fasting when you see it, and if clouds or darkness prevent you from seeing it, then complete the number of days of Shaban, and do not fast ahead of the month, and do not join Ramadan to a day of Shaban." ' (Using translation from Nasāʾī 2189)   

أَصْبَحْتُ فِي يَوْمٍ قَدْ أُشْكِلَ عَلَيَّ مِنْ شَعْبَانَ أَوْ مِنْ شَهْرِ رَمَضَانَ فَأَصْبَحْتُ صَائِمًا فَأَتَيْتُ عِكْرِمَةَ فَإِذَا هُوَ يَأْكُلُ خُبْزًا وَبَقْلًا فَقَالَ هَلُمَّ إِلَى الْغَدَاءِ فَقُلْتُ إِنِّي صَائِمٌ فَقَالَ أُقْسِمُ بِاللَّهِ لَتُفْطِرَنَّ فَلَمَّا رَأَيْتُهُ حَلَفَ وَلَا يَسْتَثْنِي تَقَدَّمْتُ فَعَذَّرْتُ وَإِنَّمَا تَسَحَّرْتُ قُبَيْلَ ذَلِكَ ثُمَّ قُلْتُ هَاتِ الْآنَ مَا عِنْدَكَ فَقَالَ حَدَّثَنَا ابْنُ عَبَّاسٍ قَالَ قَالَ رَسُولُ اللَّهِ ﷺ «صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ حَالَ بَيْنَكُمْ وَبَيْنَهُ سَحَابٌ فَكَمِّلُوا الْعِدَّةَ ثَلَاثِينَ وَلَا تَسْتَقْبِلُوا الشَّهْرَ اسْتِقْبَالًا»