Sunan an-Nasa'i, Book 22, Hadith 36 (In-Book) ⓘ

It was narrated that Ibn 'Abbas said: " I am surprised at those who anticipate the moth, when the Messenger of Allah said: 'When you see the new crescent then fast, and when you see it, then stop fasting, and if it is obscured from you (too cloudy), then complete thirty days.

أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ يَزِيدَ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرِو بْنِ دِينَارٍ، عَنْ مُحَمَّدِ بْنِ حُنَيْنٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ عَجِبْتُ مِمَّنْ يَتَقَدَّمُ الشَّهْرَ وَقَدْ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِذَا رَأَيْتُمُ الْهِلاَلَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاَثِينَ ‏" ‏ ‏.‏