Authentic
graded by Salim al-Hilali
Muwatta Malik 635
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ ثَوْرِ بْنِ زَيْدٍ الدِّيلِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم ذَكَرَ رَمَضَانَ فَقَالَ
" لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاَثِينَ "
Yahya related to me from Malik from Thawr ibn Zayd ad-Dili from Abdullah ibn Abbas that the Messenger of Allah, may Allah bless him and grant him peace, once mentioned Ramadan and said, "Do not start the fast or break it until you see the new moon. If the new moon is obscured from you, then complete a full thirty days."
Reported by Malik (No. 635) and An-Nasa’i (No. 2132).