Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2191 (DarusSalam) ⓘ

It was narrated that Simak said: "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." '

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا ابْنُ أَبِي عَدِيٍّ، عَنْ أَبِي يُونُسَ، عَنْ سِمَاكٍ، قَالَ دَخَلْتُ عَلَى عِكْرِمَةَ فِي يَوْمٍ قَدْ أُشْكِلَ مِنْ رَمَضَانَ هُوَ أَمْ مِنْ شَعْبَانَ وَهُوَ يَأْكُلُ خُبْزًا وَبَقْلاً وَلَبَنًا فَقَالَ لِي هَلُمَّ ‏.‏ فَقُلْتُ إِنِّي صَائِمٌ ‏.‏ قَالَ وَحَلَفَ بِاللَّهِ لَتُفْطِرَنَّ قُلْتُ سُبْحَانَ اللَّهِ مَرَّتَيْنِ فَلَمَّا رَأَيْتُهُ يَحْلِفُ لاَ يَسْتَثْنِي تَقَدَّمْتُ قُلْتُ هَاتِ الآنَ مَا عِنْدَكَ قَالَ سَمِعْتُ ابْنَ عَبَّاسٍ يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ حَالَ بَيْنَكُمْ وَبَيْنَهُ سَحَابَةٌ أَوْ ظُلْمَةٌ فَأَكْمِلُوا الْعِدَّةَ عِدَّةَ شَعْبَانَ وَلاَ تَسْتَقْبِلُوا الشَّهْرَ اسْتِقْبَالاً وَلاَ تَصِلُوا رَمَضَانَ بِيَوْمٍ مِنْ شَعْبَانَ ‏" ‏ ‏.‏