Sunan an-Nasa'i, Hadith 2122 ⓘ
It was narrated form Ibn 'Umar that the Prophet said: "D not fast until you see it, and do not stop fasting until you see it, and if it is obscured from you (too cloudy), then work it out."
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا يَحْيَى، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ، قَالَ حَدَّثَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لاَ تَصُومُوا حَتَّى تَرَوْهُ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ " .
also referenced under: Book 22, Hadith 33 (In-Book) & Volume 3, Book 22, Hadith 2124 (DarusSalam)