Sahih Muslim 1088
Sahih Muslim · Book 13: The Book of Fasting · Clarifying that it does not matter whether the crescent is large or small, for Allah, the Most High, causes it to appear for long enough that people can see it, and if it is cloudy then thirty days should be completed
Abu'l-Bakhtari reported:
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا غُنْدَرٌ ، عَنْ شُعْبَةَ ، ح وَحَدَّثَنَا ابْنُ الْمُثَنَّى ، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ ، أَخْبَرَنَا شُعْبَةُ ، عَنْ عَمْرِو بْنِ مُرَّةَ ، قَالَ سَمِعْتُ أَبَا الْبَخْتَرِيِّ ، قَالَ أَهْلَلْنَا رَمَضَانَ وَنَحْنُ بِذَاتِ عِرْقٍ فَأَرْسَلْنَا رَجُلاً إِلَى ابْنِ عَبَّاسٍ - رضى الله عنهما - يَسْأَلُهُ فَقَالَ ابْنُ عَبَّاسٍ - رضى الله عنهما - قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّ اللَّهَ قَدْ أَمَدَّهُ لِرُؤْيَتِهِ فَإِنْ أُغْمِيَ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ " .
We saw the new moon of Ramadan as we were at Dhit-i-'Irq. We sent a man to Ibn Abbas (Allah be pleased with both of them) to ask him (whether the sighting of a small moon had something of the nature of defect in it). Upon this Ibn 'Abbas (Allah be pleased with both of them) said that the Messenger of Allah (ﷺ) had said: Verily Allah deferred its sight, but if (the new moon) is hidden from you, then, complete its number (thirty).