Yahya related to me from Malik from Thawr ibn Zayd ad-Dili from Abdullah ibn Abbas that the Messenger of Allah ﷺ 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."
أَنَّ رَسُولَ اللَّهِ ﷺ ذَكَرَ رَمَضَانَ فَقَالَ لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاَثِينَ