Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2118 (DarusSalam) ⓘ

It was narrated that: 'Abdur-Rahman bin Zaid bin Al-Khattab addressed the people on the day concerning which there was doubt (as to whether the month had begun) and said: "I sat with the Companions of the Messenger of Allah and asked them, and they a narrated that the Messenger of Allah said: 'Fast when you see it and stop fasting when you see it, and perform the rites on that basis. If it is obscured, then complete thirty days, and if two witnesses testify then fast and stop fasting."

أَخْبَرَنِي إِبْرَاهِيمُ بْنُ يَعْقُوبَ، قَالَ حَدَّثَنَا سَعِيدُ بْنُ شَبِيبٍ أَبُو عُثْمَانَ، - وَكَانَ شَيْخًا صَالِحًا بِطَرَسُوسَ - قَالَ أَنْبَأَنَا ابْنُ أَبِي زَائِدَةَ، عَنْ حُسَيْنِ بْنِ الْحَارِثِ الْجَدَلِيِّ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ زَيْدِ بْنِ الْخَطَّابِ، أَنَّهُ خَطَبَ النَّاسَ فِي الْيَوْمِ الَّذِي يُشَكُّ فِيهِ فَقَالَ أَلاَّ إِنِّي جَالَسْتُ أَصْحَابَ رَسُولِ اللَّهِ صلى الله عليه وسلم وَسَاءَلْتُهُمْ وَأَنَّهُمْ حَدَّثُونِي أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ وَانْسُكُوا لَهَا فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا ثَلاَثِينَ فَإِنْ شَهِدَ شَاهِدَانِ فَصُومُوا وَأَفْطِرُوا ‏" ‏ ‏.‏